Davening Questions

Case: There is a minyan of 10 people. Nine, including the shaliach tzibur, have already davened Shemoneh Esrei. The 10th has not yet davened Shemoneh Esrei.

(Some questions have corresponding sources)

1) Can the 10th person daven together with the shaliach tzibur at the same time and be counted as part of the minyan?
2) Should the 10th person serve as the shaliach tzibur? What if he does not know how to?
3A) Is the requirement to have kavanah, to answer amen, or both?
3B) Is davening together with the shaliach tzibur considered kavanah?
4) I heard that if 6 people have finished davening and 4 are still davening, the shaliach tzibur can begin Chazarat HaShatz. Is this true? Is there a source? Does this apply to every tefilah?
5) Is there a clear source that states that if the 10th person davens together with the shaliach tzibur, he is counted as part of the minyan?

Sources I have seen:

Siman 109:2
1) If one begins to daven with the shaliach tzibur, when he reaches Kedushah with the shaliach tzibur he should say it with him word for word, and he should also say Birkat HaKel HaKadosh and Shomea Tefilah, and similarly by Modim so he can bow for Modim with the shaliach tzibur. This is the concept that one can daven together with the shaliach tzibur (there is a machloket about the level on which this is considered...).

Siman 124:2
2) A shaliach tzibur who comes into shul and finds that the tzibur has already davened the silent Shemoneh Esrei should not daven the silent Shemoneh Esrei, but should instead begin Chazarat HaShatz, and he does not need to daven the silent Shemoneh Esrei afterward.

Siman 124:4
3A–3B) When the shaliach tzibur says Chazarat HaShatz, the congregants must be quiet, have kavanah for the berachot, and answer amen. If there are not 10 people having kavanah for the berachot, his berachot are close to being levatalah. Therefore, each person should imagine that there are not 9 others without him, and he should have kavanah for the berachot of the chazzan. This shows that kavanah is the main part and that 9 people with kavanah are needed.

Practical case: This happened in yeshiva. We were more than 10, but many were before Shemoneh Esrei or in the middle. There was one bachur who was ready to daven Shemoneh Esrei together with the shaliach tzibur, and I mentioned that it is not a problem and that the shaliach tzibur can start. The shaliach tzibur ended up waiting until 10 had fully finished davening. I am looking for a source for this concept.

Shalom!

Thank you for your question.

If I understand correctly, here are the answers:

1. Generally,  While not ideal, if there was a minyan in the room when 9 davened, this 10th person who, for whatever reason, did not daven can daven along with the Shaliach Tzibbur, However in your case since there wont be 9 people answering Amen he may not start Chzarat Ha'shats.

2. The consensus of the poskim is that he should NOT serve as shaliach tzibuur, however if like in your case there won't be 9 people answering amen then he may be shaliach tsibbur so there will be 9 people answering Amen(Mishna Berura 124:6)

3a - The congregation is to listen attentively and answer Baruch hu uvaruch shemo and Amen, and the one reciting together with the shaliach tsibbur may only join the congregation with saying the kedushaha and the amen to the three brachos of the birkat Kohanim. (however not amen on the bracha itself that the Kohanim recite before Birkat Kohanim)

3B - This is considered a proper Shemoneh Esrei when done properly. 

4. One should always wait for 9 to have finished, before starting Chazarat Hashats, as brought in the Shulacha Aruch 124 4

5. as we explained he cannot be counted as the one on the none who need to be able to answer Amem.

Regarding your case in Yeshiva: As you yourself pointed out, the Chazzan should best not begin until 9 have completed their Shemone Esrei.

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