The Laws of the Kaddish vol.4, no. 26
The Kaddish may only be recited in the presence of a quorum of ten adult males.1 Ten men who do not understand the language of the kaddish may recite it in the language they understand.2 If less than ten men began learning or reciting Psalms ( for instance Ashrei before Mincha or Selichos ) and afterwards a quorom developed, they should recite three verses and then the Kaddish.3
One should respond to the Kaddish with focus and concentration upon the words he is saying. He should not even be thinking Torah thoughts while answering to the Kaddish.4 It is certainly prohibited to speak while the Kaddish is being recited.5
There are varying customs in regard to the pronunciation of the text of the Kaddish.6 One should follow his custom or consult a competent halachic authority if he is unsure of his custom.
The person reciting the Kaddish should pause slightly between the words "V’imru" and "amen". When answering "Amen, yehei shmei..." one should pause slightly between "amen" and "yehei".7
One should stand when answering to the Kaddish at least until he finshes saying the words "Yehei shmei...olmayah" or until after the amen after "Yisbarach..." according to some authorities.8
If one enters the shul and hears the congregation answering to the Kaddish he should say with them "Yehei shmei..." even if he only heard the very end of the phrase. But he should not say the introductary Amen since he did not hear the words of the chazan. If he hears the majority of the congregation reciting "Amen, Yehei..." he can say also "Amen, Yehei..." if he is cognizant to what he is responding.b
Some have the custom to say "Yisbarach" after "yehei shmei...". They should pause slightly before saying it.10
1)Mishneh Brurah 55:1 2)ibid 3)55:2 4)MB 56:1 5) ibid 6)MB 56:2 7) ibid 8)MB 56:7 9)MB56:9 10) MB56:15
