The only way to learn what something sounds like as a non-native speaker is to look it up or listen to someone pronounce it. There are no rules – or at least no useful rules, because any rule will have many exceptions. Even different English dialects differ in how to pronounce words. There’s simply no making sense of it.

For example, in many British English dialects, the “a” in “can” and the one in “can’t” are pronounced completely differently, despite “can’t” being a contraction of “can not”. It’s literally the same word, just with a different word afterwords, and yet the two get different pronunciations. There’s no way to guess at that being the case, or come up with a logical reason why. You just have to accept it.