1. V + 了(right after the verb) + ... 。 ==> Short past, action finished.
wǒ chī le fàn 。
I ate and finished dinner.
2. 了 at the end of the sentence ==> Close past / change of status
tā tuì xiū le 。
He got retired (before he was not).
nǐ zhōng yú dào le 。
Finally you came.
3. V + 了 + ... + 了。 ==> Short past, action continuing.
tā chī le liǎng gè xiǎo shí le 。
He's been eating for 2 hours (and he's still eating).
Different focus point of 了 :他去了北京。
tā qù le běi jīng 。
He went to Beijing (but we don't know if he is back or not).
--> This action of going is finished.
tā qù běi jīng le 。
--> Focus on the change of status. Before he was here, but now he has moved there.
tā qù le běi jīng le 。
--> This action happened in the past and is still going on.
· at last
· in the end
· to participate
· to take part