Sukkah, Chapter 3, Mishnah 10. This mishnah deals with a person who does not know how to read Hebrew and cannot recite Hallel on his own, and with the question of how he fulfills his obligation.
One who does not know how to read, and who reads for him:
A person who does not know how to read Hebrew and wishes to recite Hallel has someone read before him, and this is the language of the mishnah, "makrin oso" - they read before him. The mishnah lists three:
His slave.
His wife.
A minor - one who has not yet reached the age of bar mitzvah.
None of these are obligated in the recitation of Hallel, and for that reason they cannot fulfill his obligation for him. They cannot have him in mind and activate the principle of "shome'a ke'oneh" - whereby a person listens to someone else who is obligated in the mitzvah, and it is considered as though he said it himself. This principle does not operate with them, since they themselves are not obligated in the mitzvah of Hallel.
Therefore the mishnah rules that he must repeat after them everything they say. They guide him to say the correct text, but he is the one pronouncing the words himself, and in this way he fulfills the mitzvah on his own. They are not helping him fulfill the mitzvah directly, but only by leading him and supplying him with the correct words to say.
Nevertheless, the mishnah comments here, "tavo alav kelalah" - a curse should come upon him for never having troubled to educate himself and learn how to read Hebrew, something he was obligated to do.
When an adult who is obligated in Hallel reads before him:
In this case he is not required to say every single word of Hallel himself; it is enough that he respond after each section and say "Halleluyah". This response parallels the saying of "amen" after a berachah: in Hallel, the parallel response is saying "Halleluyah". By this he participates and expresses his desire that those words be considered as though he said them himself, and he fulfills his obligation in the mitzvah of Hallel through the other person's recitation.
In summary: One who does not know how to read, and whose slave, wife or a minor reads before him - people who are not obligated in the mitzvah and cannot fulfill his obligation through "shome'a ke'oneh" - must repeat after them word for word, and a curse should come upon him for not having learned to read. But if an adult who is obligated in Hallel reads before him, it is enough for him to answer "Halleluyah" after each section, and he fulfills his obligation through the other person's recitation.