I was reading surah yasin and in ayah 12 in the third word I pronounced the ya with a kasra--so nuhyi'lmauta--but my..

Assalaolaikum wa rahamatullah
Today in my tajweed lesson I was reading surah yasin and in ayah 12 in the third word I pronounced the ya with a kasra--so nuhyi'lmauta--but my teacher said it is pronounced nuhiillmauta--(it is difficult to type phonetically)-- but can you explain why we do not pronounce the ya clearly with a kasra here...and instead seem to join the ha with the ya then put a kasra under this merged letter/sound...I hope this makes sense inshallah.
masalaama

Answer

Wa alaikum assalaam wa rahmatullahi wa barakatuh.

It is difficult to be able to determine with transliteration what was pronounced incorrectly so that the teacher corrected you. It may be that you somehow pronounced it with a shaddah or put some sort of emphasis on the  other than the correct sound, we cannot tell by the transliteration. 

The word  does have a kasrah on the  but with no shaddah, and this   is pronounced clearly.  It is not joined with the , but the has the running of the sound and breath with it, so when saakinah, has more timing than some of the other letters.   When reading the combination , one reads the  with running of the sound and breath and then should go directly from the  with the kasrah to the  saakinah, with no lengthening of the letter .  

It may be best if you listen to a recording of Sheikh Al-Husary or Sheikh Abdullah Basfar and listen how they pronounce this word combination and compare it with how you read it.   These links have a page of listings of surahs for both of them with recitation of the Qur’an, and you will need to scroll down to surah Ya Seen, then click on either the MP3 or Real Player, depending the set up on your computer.

http://english.islamway.com/bindex.php?section=echapters&recitor_id=24

http://english.islamway.com/bindex.php?section=echapters&recitor_id=92

May Allah increase you in beneficial knowledge and specifically knowledge about tajweed.
Wa assalaam alaikum wa rahmatullah.