In what cases must I use 了 and in what cases not? 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!
In what cases must I use 了 and in what cases not?



You use 了 when you must indicate the verb is 'completed'
you don't need 了 when you don't need to indicate the verb is completed


From my answer to another post


  The verb particle 了 denotes the "completion" aspect of the verb
  
  
  经历風浪 = [going through crisis] (In this sentence, the verb 经历 (to experience) may or may not had been completed
  经历 (了) 風浪 indicates [the verb 经历 in "experience crisis" is completed]
  


One more example:


吃水餃 = eat dumplings (we don't know you are going to eat dumplings, in the process of eating dumplings or done eating dumplings )


The proof is 吃水餃 can be the answer for any of the following questions:

Q1: "你現在要幹什麼?" (What are you going to do now?)

A1: "吃水餃" (or 我現在要吃水餃)

Q2: "你在幹什麼?" (what are you doing?)

A2: "吃水餃" (or 我在吃水餃)

Q3: "你幹了什麼?" (What have you done?)

A3: "吃水餃" (or 我吃了水餃)


吃了水餃 = has eaten dumplings (we know the verb 吃 is completed, you have done eating)


It can only be an answer to Q3, but not Q1 or Q2

Even more example:

If you only say "做菜請客", it doesn't matter if you are in the middle of cooking or the cooking is done, the sentence would be correct; If you say "做了菜請客", it can only mean the cooking is done and the dishes are ready

This answer focus on the relationship between verb and the verb particle了.

了 as a final particle has other functions. It denotes a sentence has ended; indicating change of situation,  ; serve to soften the tone of a sentence; express different emotions in speech depend on pitch and tone