chinese grammar "了"

Who can explain me the use of "了"

There are many uses of 了.  The most important one is that it is an indicator of past.  This is how Chinese expresses past tense.

他吃了 he HAD eaten, or he ATE already.  In English, using present tense, "He eats" is not expressed in Chinese as "他吃".  It would sound strange to Chinese ears.  It must be in some other context.  For example, "He eats like a pig" would be 他吃得像一隻猪.

The other common meaning is "done", "finished".  For example. 不了情, "unfinished love", or "unending love".  Here the pronunciation changes from "le" to "liao".