Amen

From Halachipedia
  1. If a person hears a Bracha made properly by a Jew there’s a obligation to answer Amen. (The obligation is based on the פסוק of כי שם יהוה אקרא הבו גדל לאלהינו). [1]
  2. It’s proper to answer Amen after a Tefillah or Bracha even if it doesn’t have the name of Hashem (Shem UMalchut) for example: the Mi SheBerach (מי שברך), Harachaman (הרחמן) in benching, and some add “Makom Yenachem Etchem…”. [2]
  3. The proper intent one should have when saying Amen is “ the Bracha being made is true and I believe in Hashem”. For a Bracha of a request or Tefillah one’s intent should include “it’s should be Hashem’s will that it comes true.” [3]
  4. Amen must be said within Toch Kedi Dibbur (2-3 seconds) of the Bracha or if one’s in a congregation one can say Amen until the majority of the congregation has finished saying Amen. [4]
  5. When hearing a Bracha over a microphone, if one’s in the same room one may answer Amen, if not don’t answer. [5]
  6. One may answer amen to a Yehi Ratzon a Jew says even if the Yehi Ratzon didn't have Hashem's name in it.[6]

Amen Yetomah

  1. If one is obligated in a certain Bracha and one didn’t hear the Bracha it’s forbidden to answer Amen (which is called in halacha Amen Yetomah). If one is not obligated in the mitzvah as long as one knows which Bracha is being made it’s permissible to answer Amen. [7]
  2. Preferably one should hear the Bracha of the Shaliach Tzibbur and answer amen, however, after the fact, even if one didn’t hear the bracha but as long as one knows which Bracha was made, one may answer amen. [8]

Answering to Kaddish

  1. One who answers amen yehei shema rabba with all his strength annuls any bad decrees. [9]

References

  1. S”A 215:2, Mishna Brurah 215:8, Rambam Hilchot Berachot 1:13, Kaf Hachayim 124:30
  2. Mishna Brurah 215:9, Vezot HaBracha pg 188 in name of Rav Shlomo Zalman
  3. Mishna Brurah 124:10, Vezot HaBracha pg 188)
  4. Rama 124:8 says that the Amen must be said immediately after the Bracha and Mishna Brurah 124:34 explain it means Toch Kedi Dibbur. Vezot HaBracha (pg 189) agrees.
  5. Vezot HaBracha (pg 189) in name of Rav Shlomo Zalman
  6. Nitai Gavriel Rosh Hashana 29:23
  7. S”A 124:8 holds that only is a person was obligated in a Bracha and didn’t hear it, even if one knows which Bracha is being made, one can’t answer Amen. However, if one wasn’t obligated in the Bracha, one wasn’t obligated in a Bracha, one may answer as long as one hears others saying Amen even if one doesn’t know which Bracha was made. Yet, Rama argues that even by Brachot that one’s not obligated in one shouldn’t answer Amen if one doesn’t know which Bracha was made. However, if one knows which Bracha is being made one may answer amen if one’s not obligated in the Bracha.
  8. Concerning Chazarat HaShas the Mishna Brurah 124:33 writes that it’s preferable to hear the Bracha from the shaliach tzibbur since some say that since it’s a rabbinic institution it’s like it’s a Bracha one’s obligated in, however, after the fact, one can answer as long as one knows which Bracha was made (because one knows which Bracha the tzibbur was up on even though one didn’t hear the bracha). Vezot HaBracha pg 189 agrees. Yalkut Yosef (Tefillah vol 2 pg 163, siman 124 note 8) rules that one should be strict like the Rama, except that by Chazarat HaShas if one already prayed one may answer as long as one knows which Bracha is made.
  9. Gemara Shabbat 199b)