Question
Assalaamu aleykum,

I have difficulty reciting "kama ya'rifuuna abnaa'hum" in 6:20 (Sura al-An'aam). There is a dhamma on the miim of abnaa'hum and at the same time there is a stop sign "miim". So should I stop reciting abnaa'humu or abnaa'hum?

Jazaakallahu khairan!

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The dhammah on the meem is for when continuing only.  The Arabs normally stop with a sukoon (no vowel) on the last letter.  The first pronounced letter of the next word is saakin, so the meem acquires a dhammah when continuing in order to eliminate two saakin letters being read one after another, something not done in Arabic.   The plural meem in inheritanly saakin, but acquires a dhammah under this circumstance. 
Please also see:
http://www.abouttajweed.com/121003.htm

Wa iyyaakum