What are the three categories of brochos in Shemoneh Esrei?

Shemoneh Esrei is grouped into three categories: the first three brochos, the middle brochos, and the final three brochos. The first three brochos are, to some extent, considered one brocha, and the same is true for the last three brochos. Therefore, if a person makes a mistake or omits a critical addition in one of these brochos, he must repeat the entire set. This is why a person who omitted Morid Hageshem and continued onward must return to the beginning of Shemoneh Esrei and not just the beginning of the second brocha. The middle brochos, however, are exclusive of one another. If one makes a critical mistake in one of these brochos, he simply has to go back to that brocha. Nevertheless, because the brochos must be recited consecutively, one who goes back to a brocha after a mistake or omission must continue with the brochos that come after it. If he would only repeat the original brocha, Shemoneh Esrei would not be recited in order.

סי' קי"ט ס"ג ס"ק י"ג-ט"ו

Does the chazan go back if he omitted Aneinu?

During Chazoras Hashatz on a Ta’anis Tzibur, the chazan adds an additional brocha of Aneinu. This brocha is inserted between the brochos of Geulah and Refuah. If it was omitted entirely, it is not necessary to repeat Chazoras Hashatz. If the chazan forgot to say it and he has not yet said the name of Hashem at the end of Refoeinu, he should go back to Aneinu. Otherwise, he should continue onward and say Aneinu as part of Shema Koleinu. If he were to go back to Aneinu, he would have no choice but to repeat Refoeinu (otherwise, the brochos would not be recited in order). Since Aneinu is not a critical addition, unnecessarily forcing oneself to repeat Refoeinu creates a brocha l’votolah.

ס"ד ס"ק ט"ז-י"ז

For Aneinu to be said, how many people in the minyan must be fasting?

In order to say Aneinu during Chazoras Hashatz, ten people in the minyan must be fasting. According to some Achronim, this only applies to a ta’anis which was instituted for a specific calamity. For the four standard fasts, however, it is sufficient if there are just seven people in the minyan who are fasting. According to some opinions, the people who are fasting do not actually have to be davening in that minyan. It is sufficient if they are in shul during Chazoras Hashatz. It is not necessary to ask the people in shul if they are fasting, since it can be assumed that most people fast on a standard ta’anis.  

ס"ק י"ח ביאורים ומוספים דרשו 23-24

• One may make requests that pertain to immediate needs during the brochos relevant to those needs. Thus, if a member of one’s household is sick, he is permitted to make an additional request during the brocha of Refoeinu. Requests that pertain to possible future needs (e.g. prayers to prevent illness) are forbidden at that time.

• In the brocha of Shema Koleinu one may request anything, regardless of topic or immediate relevance. If one does have a specific need, he should make the request in the pertinent brocha and not wait for Shema Koleinu.

• It is recorded that when one of the Rishonim fell ill, the tzibur gathered for a ta’anis and extra tefillos and they added a lengthy supplication to the brocha of Selach Lonu. Tefillos on behalf of a gadol b’Yisroel whose Torah is critical for the entire nation are considered a public need for which lengthy tefillos are permitted.

• What is the meaning of the words ishei Yisroel?

• When is the appropriate time to say Yihyu L’rotzon?

• May one learn before he takes three steps back?