已经【yǐjīng】

已经 means "already". It goes before the verb, and the sentence nearly always closes with 了.

The 了 is doing the work of saying the situation has changed. 已经 only marks that the change came sooner than expected, or sooner than the listener knew.

With a quantity

In the negative

已经不 for something no longer true, 已经没 for something no longer there.

Position

已经 is an adverb, so it goes after the subject and before the verb. yǐ jīng已经wǒ我chī吃le了 is wrong.

Questions

The answer drops 已经: 吃了.

已经 against 都

都 plus a time also means "already", with surprise or complaint attached: 都十点了 sounds impatient where 已经十点了 merely reports.