Verbs
了 marks completion, not the past
了 after a verb says the action is finished. When it happened comes from the time word, not from 了.
Chinese has no tenses, so nothing in a verb tells you when something happened. 了 says only that the action is complete, which is why it appears in sentences about the future as readily as the past. At the very end of a sentence it does a second job: it marks a change, saying that something has become true which was not true before.
Examples
我吃了wǒ chī leI've eatencompletion, straight after the verb
我买了两本书wǒ mǎi le liǎng běn shūI bought two books了 goes before the object
下雨了xià yǔ leit's started rainingend of sentence, so a change
我没吃wǒ méi chīI haven't eaten没 for the negative, and no 了
Watch out
The mistake this causes
Saying 我没吃了. 了 and 没 never appear together, because 没 already says it did not happen.