I just wrote a whole explanation but it disappeared. =:^(

So I will just say that, judging from your post, you appear to be at the beginning level of your Russian studies. There are rules for what you ask, but then each would have to be presented in a fair amount of detail, which would be beyond your level at this point. I would just use the information you find in your textbook and accompanying CD or DVD to learn Russian pronunciation. Also, you might look in the grammar part of masterrussian for some presentation of pronunciation. Who says Russian is pronounced like it is spelled? =:^) Good luck!