Sorry if we missed your question earlier. You're right, he letter alif can be pronounced in either a "thick" way, as in [rocket-record phraseId="55282"]
or a "light" way, as in [rocket-record phraseId="73059"]. It usually depends on the letter that comes before the alif, but of course there are some exceptions, and the rules are different than those that apply for MSA. In general, the letters that make the alif sound light are
ت - ث - ج - خ - د - ذ - ز - س - ش - ع - غ - ف - ك - ل - ي
The letters that make the alif sound heavy or thick are
ب - ح - ر - ص - ض - ط - ظ - ق - م - ن - هـ - و
It is difficult to memorize them, but you will get used to it by practice.