In Bentching by the end of the Harachamon's we say וְנאמַר אָמֵן and at the end of Magdil we say ואמרוּ אָמֵן. Is there a difference? What is the difference? Why the difference?
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See Mateh Moshe (2:338), who brings from the Mordechai that one should say "ואמרו אמן" at the end of the Harachaman section as an exhortation (אזהרה) to the others there to answer "amen" as a form of necessary respect to the host, similar to the end of the "Magdil" section, where we do the same as a form of necessary respect toward Hashem (the object of our praise in that section).
Mishna B'rura (189:5), on the other hand, draws a distinction between the two instances, in that the latter is involved with the praise of Hashem ("...עושה שלום"), which is obligatory, and so warrants the use of "ואמרו אמן". The former though, is not obligatory (at least to the same level) and thus Mishna B'rura prefers the use of "ונאמר אמן".
Generalizing from this, it seems that "ואמרו אמן" is used when everyone is absolutely required to respond; it is used as a sort of command. Whereas "ונאמר אמן" is more like "And let us say..." in a more relaxed fashion.
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In the last harrahcaman (...הָרַחֲמָן הוּא יְבָרֵךְ אֶת בַּעַל הַבַּיִת הַזֶּה) we are asking for Hahsem to bless us. We use the future tense, ונאמר, to mean that when Hashem blesses us, we will say אמן.
We then use the imperative form, ואמרו, after עושה שלום, to tell everyone else to say אמן to our wish that עושה שלום במרומיו (ה'), הוא יעשה שלום עלינו ועל כל ישראל. Admittedly, this presents a problem with the meaning of ואמרו when benching by yourself and at the end of shmoneh esre when no can hear you. It would be difficult to say that you are telling someone else to say אמן in these cases.