You use 'ga' when you're talking about the grammatical subject, but with 'ha' this isn't always the case. (For example, you can give direct objects the 'ha' particle).
Or, when you want to talk about two things that are not the same (... and I just forgot the common English word for this...)
But usually, ha roughly translates to 'the', while ga is 'a' (I thought that the English term for this is defined vs. undefined article, but I'm not sure)
The rest have easier basic rules, but some verbs have specific particles that they always go with. But you should just learn those as set expression.
Speak: Dutch, English
Something between reading and speaking: French, German
Read: Latin, Ancient Greek
Japanese: somewhere around JLPT 4 level (out of 5), I guess. Also, I (should) know around 250-300 kanji.