了【le】

了 at the end of a sentence says something is different now from how it was. It is not a past tense.

我饿 on its own is barely a sentence. 我饿了 reports a change: I wasn't, and now I am.

With an adjective

With a number or a quantity

不…了 for stopping

The 了 is what makes it a change of plan. 我不去 is a refusal; 我不去了 says the plan has been dropped.

没有…了 is wrong

A change that did not happen simply takes 没. wǒ我méi没chī吃le了 is wrong; say 我没吃.

太…了

了 closes this pattern too and carries the same sense of a threshold crossed.