We are in Maseches Berachos, chapter 9, mishnah 3. The mishnah opens with the law of "banah bayis chadash v'kanah keilim chadashim" - someone who built a new house, bought new vessels or new garments makes a berachah. From here the mishnah returns to the berachos discussed in the previous mishnah: the berachah of someone to whom something good has happened, and the berachah of someone to whom something bad has happened.
One makes a berachah on the bad and on the good according to the moment of the event:
On the bad: A person makes a berachah on the bad things that happened to him, even though in the end they will turn out to be good. For example, someone whose field was flooded with water - at this moment the flood is entirely bad, since he has lost his entire crop, even though ultimately his field will be especially fertile because of the irrigation it received. Since at present the matter is bad, he makes the berachah for bad news and not a different berachah.
On the good: And so too the reverse, with something good out of which a bad situation may develop. For instance, someone who found a valuable object that he is permitted to keep for himself, but he is worried that the government will hear about it, take it from him, and even punish him for not having handed it over to them in the first place. At this moment he is not required to worry about the future, and at present the matter is good - and therefore he makes the berachah of "HaTov".
One who cries out over the past - a wasted prayer:
Here the mishnah teaches a fundamental principle regarding prayer: someone who cries out to Hashem and prays about something that has already happened - even though the facts are not yet known to him, but he knows that the thing has already taken place - his prayer is a wasted prayer, "harei zu tefillas shav". The reason is that the Holy One, Blessed be He, does not change what has already been done.
The Gra notes that this law is not limited to something that has already happened: even a request for something that cannot occur by natural means, but only miraculously, falls under the category of a wasted prayer. The examples the mishnah chose, however, deal with something that has already happened:
"yehi ratzon shetelaid ishti zachar" - when his wife is already pregnant, the child is already either male or female, and therefore this is a wasted prayer.
"shelo yihyu eilu bnei beisi" - a person who is traveling on the road and hears a cry of distress in the city, and knows that something bad has happened in one of the houses, and says: may it be the will of Hashem that these not be the members of my household - this too is a wasted prayer, since the matter has already occurred.
A difficulty with this halachah:
The Gemara in Maseches Taanis states: "a blessing is found only in something hidden from the eye" - blessing rests specifically on things concealed from the eye, in which the Holy One, Blessed be He, is able to place blessing; but once the matter has been revealed, changing it requires an open miracle. On the face of it this contradicts the halachah of our mishnah, for in the mishnah's examples we are dealing with something that has occurred but has not yet become known and revealed.
Interestingly, the Melechet Shlomo on this mishnah brings the commentary of the Mizrachi on Rashi in the Chumash in Parashas Vayetzei, as well as other statements of Chazal, from which it emerges that even up until the very moment of birth a person may pray that his wife give birth to a male, and this is not considered a wasted prayer.
It appears, then, that Chazal disagreed about this: something that has already been determined in reality but is still not known to the person and not revealed to the public - is it within the power of the Holy One, Blessed be He, to change it, and is prayer about it considered a prayer for a miracle or not?
In summary: In this mishnah we learned that a person's berachah is determined according to the present situation - he makes the berachah on the bad at the time that it is bad, even though it will end in good, and he makes the berachah of "HaTov" at the time that the matter is good, even though bad may develop from it. We also learned the principle that "one who cries out over the past, this is a wasted prayer", and the Gra's extension of this to any prayer for something that is not possible by natural means. Finally we examined the difficulty from the Gemara in Taanis, and the approach of the Mizrachi cited by the Melechet Shlomo, from which it emerges that regarding something not yet revealed, Chazal disagreed as to whether there is room for prayer.