Siman 489, Se'if 3

Shulchan Aruch, Siman 489, se'if 3
"One who davens with the congregation while it is still day counts along with them without a berachah, and if he remembers at night, he recites the berachah and counts."
(Shulchan Aruch, se'if 3)

The practical case this se'if is dealing with

Shuls that daven Maariv early: There are shuls that attach Maariv to Minchah so that the crowd does not disperse between the two tefillos, and as a result they end up davening Maariv while it is still daytime.
The visitor who happens to be there: A Jew who finds himself in such a shul has a problem, because at the end of Maariv they count sefiras ha'omer, and it is still early.
The Shulchan Aruch's advice: He should count along with them without a berachah, and if he remembers later at night, he recites the berachah and counts again.

The basic difficulty in the Shulchan Aruch's words

Counting without a berachah: Counting without a berachah is always permitted and raises no concern. A person may even count several times a day, as long as he does not recite a berachah, and there is nothing wrong with that.
One who has already fulfilled a mitzvah does not go back and bench: The simple rule is that a person who performed a mitzvah properly but did not recite the berachah has lost the berachah and cannot recite it afterward.
An example of this rule: Someone who blew shofar properly and forgot to recite the berachah beforehand has fulfilled the mitzvah, and he cannot recite the berachah once he remembers, no matter why he forgot.
Hence the question: How can the Shulchan Aruch rule here that even though he already counted, he recites the berachah and counts again at night? This is an unusual chiddush, and therefore we need to understand what it rests on.

The Mishnah Berurah (s.k. 7): what "while it is still day" means

"While it is still day: some of the Acharonim explained that this means bein hashemashos, which is a doubtful nightfall, and one fulfills the counting then according to the poskim who hold that sefirah nowadays is only a rabbinic obligation."
(Mishnah Berurah, s.k. 7)

It does not mean literally daytime: We are not speaking of someone counting at three in the afternoon; such a count accomplishes nothing at all. The reference is only to bein hashemashos, the period of twilight.
The definition of bein hashemashos: It is uncertain whether it is day or night. If it were definitely night, obviously one could count; the entire discussion concerns this state of doubt.
A doubt in a rabbinic law is resolved leniently: If sefiras ha'omer nowadays is a rabbinic obligation, we resolve the doubt leniently and treat the time as night, and therefore one can count.
Were sefirah a Torah obligation: In a doubt concerning Torah law we rule strictly, and we would have to treat the time as day, in which case this counting accomplishes nothing, since it is still being done on the previous day.
The Shulchan Aruch's position here: The Mishnah Berurah explains that the Shulchan Aruch is following the view that sefiras ha'omer nowadays is rabbinic, and for that reason one may count during bein hashemashos.
The phrase "some of the Acharonim": The Mishnah Berurah is precise in his wording, since this explanation is not universally accepted; he is explaining the se'if according to this approach.

Why should he count with them already at bein hashemashos?

Those who are careful: We explained earlier that those who are meticulous do not count at bein hashemashos but wait until it is definitely night. This Jew is used to counting at night, so the question arises: why should he count with them at the earlier time?
Lest he forget: The main reason is that if he does not count with the tzibbur, there is a concern that he will get home and forget to count altogether.
People remind one another: When davening with a minyan, one person reminds the next, and in a tzibbur people do not forget to count the omer.
"In a multitude of people is the King's glory": We explained in earlier shiurim that there is a special value in counting the omer together with many people.
The harvesting of the omer before a large crowd: The Mishnah describes that the omer was harvested in the presence of a large crowd, because this was a symbolic act against the Tzedukim, who began the counting on Motza'ei Shabbos. Publicizing the matter before many people made known that the harvesting and the counting begin on the night following the first day of Pesach. From here we see that there is value in the counting itself being done with a tzibbur as well.

A condition attached to a mitzvah

"And the Mechaber said that he should count with them lest he forget afterward, and he should think in his mind: if I remember later at night to count, I do not wish to fulfill my obligation with this counting; and therefore when the time arrives later he recites the berachah and counts."
(Mishnah Berurah, s.k. 7)

The name of the device: This mechanism is called a "condition attached to a mitzvah." A person performs the mitzvah while stipulating in his mind that if he gets the opportunity to perform it at the preferred time, he does not wish to discharge his obligation with the first act.
Its application here: He counts with the tzibbur at bein hashemashos, and stipulates that if he remembers at home after tzeis hakochavim, whatever he counted in shul is void, and then he will recite the berachah and count properly.
The dispute: Not all the poskim accept this idea. Some argue that one should not play such games, being yotzei one time and not another; since he already counted, he has fulfilled his obligation, and consequently the berachah at home is questionable.
The weak point: If he in fact fulfilled his obligation with the first counting, why should he count again with a berachah? The whole license to recite the berachah depends on the condition actually nullifying the first counting.

Another example of a condition attached to a mitzvah: Krias Shema

The situation: In the winter the deadline for Krias Shema may be 8:15, according to the various calculations, while in shul they begin at 8:00, so the chazzan will reach Krias Shema after the time has passed.
The advice: Someone who gets up early reads the paragraph of Shema Yisrael at home before davening, without the berachos, and stipulates: if it turns out that in shul the time will pass, I wish to fulfill my obligation of Krias Shema in its proper time with this reading; and if it turns out that in shul they read it within the time, this reading should count merely as words of Torah and verses, and I will fulfill my obligation with the reading in shul, where it is read together with the berachos, which is the preferred way.
A distinction raised in the shiur: With Krias Shema the condition is made at a time when the obligation is certain, whereas with sefiras ha'omer at bein hashemashos it is still doubtful whether he is obligated at all, and from that angle the condition in sefiras ha'omer stands on firmer ground. Still, the underlying idea in both cases is the same.

The words of the Rema: answering amen to the congregation's berachah

"And even if he answered amen to the congregation's berachah, if his intention was not to fulfill his obligation, he goes back and recites the berachah and counts at night."
(Rema, se'if 3)

What is "the congregation's berachah": The Rema is following a custom of his time, in which the shaliach tzibbur would recite aloud "asher kideshanu bemitzvosav vetzivanu al sefiras ha'omer," and the congregation would answer amen without reciting the berachah themselves. This is not our practice today.
Shomea ke'oneh with berachos: With berachos the principle of shomea ke'oneh (hearing is like reciting) always applies, as with the berachah on shofar, where one person recites it and the listeners fulfill their obligation through his berachah, even though each one is obligated both in the berachah and in hearing the blasts.
The counting itself is different: With sefiras ha'omer we explained that there is a special discussion based on "usefartem lachem," that the counting must be done by each and every individual. There is a recitation and there is a counting: for the recitation shomea ke'oneh works, but for the counting it is not so simple, and not all poskim hold that one who hears "today is five days of the omer" has fulfilled his obligation.
The difficulty with the Shulchan Aruch's advice: Seemingly this Jew heard the berachah from the shaliach tzibbur and answered amen, so he has discharged his obligation with that berachah and can no longer recite it at home.
The Rema's answer: Therefore it is not enough for him to count without a berachah; he must also have in mind, before answering amen, that he does not wish to fulfill his obligation with that berachah, and then he goes back and recites the berachah and counts at night.

Mishnah Berurah (s.k. 10): specifically when he intends not to be yotzei

"Not to fulfill his obligation: and specifically if he intends not to fulfill it, but if he had no particular intention..."
(Mishnah Berurah, s.k. 10)

With no particular intention: A person who heard the berachah and neither intended to fulfill his obligation nor intended not to, according to many poskim has fulfilled his obligation, and the very act of answering amen suffices. This depends on the question of whether mitzvos require intention, as explained in Siman 60, and there is a major dispute about it, with some holding that for a rabbinic berachah no special intention is needed.
Therefore he must have explicit intention: Since with no particular intention he may well have fulfilled his obligation, he is instructed to specifically intend not to be yotzei, so that it is clear he did not discharge his obligation with the shaliach tzibbur's berachah.
The concern of a berachah levatalah: The discussion here touches on Hashem's Name. If we say that he did fulfill his obligation with that berachah and he nevertheless recites it again at home, he has transgressed "You shall not take the Name of Hashem your God in vain," and because of this concern we insist on explicit intention not to be yotzei.
The conclusion: One who explicitly intended not to fulfill his obligation certainly did not fulfill it, and therefore when he comes home after tzeis hakochavim he can recite the berachah and count with a clear mind.

In passing · The titles of Rav Yosef Karo and his rules of decision

The Mechaber: The scholars refer to Rav Yosef Karo as "the Mechaber," the author, because he authored the Shulchan Aruch, and that is the term the Mishnah Berurah uses here.
The Mashbir: Another title, based on the verse about Yosef, "He was the provider for all the people of the land." It generally refers to his work the Beis Yosef, which is like an encyclopedia supplying every student with all the opinions. The Beis Yosef is the work that preceded the Shulchan Aruch, and in his own day Rav Yosef Karo was famous precisely because of the Beis Yosef, while in later generations the Shulchan Aruch became the primary sefer.
Maran: A title of honor, and it contains a hint: Maran, an acronym for "two hundred rabbanim endorsed him," meaning that he received the approbation, as it were, of two hundred rabbanim. This title was given to him in order to strengthen the authority of his words, because there were those who challenged the Shulchan Aruch in his time. (The title Maran was given to other scholars in recent generations as well, but when "Maran" is used without qualification the reference is to the Beis Yosef.)

The rules of decision in the Beis Yosef and the Ashkenazi critique

The problem: The Beis Yosef brings a multitude of opinions, so the question naturally arises how the student is to know what to do in practice.
The rule he set down in his introduction: The three pillars of halachic ruling are the Rif, the Rambam and the Rosh, and when they disagree we follow the majority, two against one.
The objection raised against him: The Rif and the Rambam are Sephardic authorities and the Rosh is an Ashkenazi authority, so the ruling of two against one will always fall to one side.
The Rema's response: The Rema wrote his glosses because he held that this determination was incorrect, and he listed additional Ashkenazi authorities who deserve to be placed in the front rank, such as the Tosafos, the Mordechai and the Semag; one should not establish that the Rif, the Rosh and the Rambam alone are the deciding authorities.