When saying something in the past tense, in what cases must I use le 了 and in what cases not?
I'm studying this right now in my Chinese class and it is a complete mindfuck. I don't know in what cases its use is compulsory, in what cases you must not use it (despite talking about past actions) and use instead some time adverbs or some other structure or just not using anything, and in what cases it is optional. It is very confusing when in every single sentence, according to a thousand factors, have to (or have not to) use le 了.
I would really appreciate if someone could list me detailed, foolproof rules of the use of le 了. I see it as something really ambiguous and diffuse and my teacher says each Chinese person uses it as they please.
Thank you so much in advance for your help!