May a person make requests for anticipated needs during Shemoneh Esrei?

There are two views among the Rishonim regarding making additions to the brochos of Shemoneh Esrei. The first view holds that in the middle of the brochos which discuss specific needs, one may make additional requests that are pertinent to the brocha. The requests can only be added after part of the brocha has already been recited. One may only make requests that pertain to immediate 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 without waiting for Shema Koleinu.

סי' קי"ט ס"א ס"ק א'-ג' ביאורים ומוספים דרשו 1

Is there a difference between making requests for individuals or requests for groups?

The second view maintains that there are more detailed instructions about making extra requests: 1) If one is making specific requests on behalf of a group of people in the middle of a brocha discussing specific needs, he must phrase it in the plural tense and he can only add it at the end of the brocha. 2) If the request is on behalf of an individual, it should be phrased in the singular tense and can be inserted any time during the brocha. 3) During the brocha of Shema Koleinu, one may add requests for both individuals and groups of people. There are some Achronim who hold that one may not make requests on behalf of groups of people, even during Shema Koleinu. Although the halacha follows the first view, one should be stringent and adhere to the guidelines of the second opinion.

ס"א ס"ק ח'-י'

Why is the health of a gadol b’Yisroel a public need?

It is best not to offer lengthy supplications on behalf of an individual in the middle of any brocha. At the end of Shemoneh Esrei, before yihyu l’rotzon, one may add any supplication he chooses, regardless of its length. 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. 

ס"ג ס"ק י"ב

• The following are the halachos for one who omitted V’sein Tal U’Matar: 1) If he realized while he was still in the middle of the brocha, he may add it at whatever point he finds himself in the brocha . If he already said the words k’shanim hatovos, he should go back, say it in its proper place, and then continue onward.

• 2) If he realized the mistake after saying Hashem’s name or after finishing the brocha, he should continue onward and say it as an additional request in Shema Koleinu. It is generally added before the words ki Ata shomeya, but on a fast day he should add it before Aneinu.

• 34) If he missed it at that point as well and only realized the mistake after he said the name of Hashem, he should say the words lamdeini chukecha and go back to the point right before ki Ata, say V’sein Tal U’matar, and continue onward. 

• What are the three categories of brochos in Shemoneh Esrei?

• Does the chazan go back if he omits Aneinu?

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