When put after the verb, 了 indicates the completion of an action

• 她三天前去了中国。She went to China three days ago. (The action 'to go' is completed.)

• 明天我到了公司就得和经理见面。I need to meet the manager once I arrive at the company tomorrow. (The second action 'to meet the manager' will take place once the first action 'to arrive' is completed.)

Note that although the English translation of a sentence with 了, such as in the first example, would naturally use the past tense, the particle 了 itself doesn’t necessarily carry a specific indication of tense. (It is in fact the expression 三天前 three days ago that explicitly indicates the past tense in the first example.)

When put at the end of the sentence, 了 indicates a change of status/situation, and emphasises the impact on the present

• 我走了。

  I'm leaving.
  (I'm moving, so my status is changed.)

• 没有了。

  No more.
  (There was some, and now there's none, so the situation is changed.)

• 我明白了。

  I see.
  (I did not understand before, but now I do, so the situation is changed.)

• 这是第二次了。

  This is the second time.
  (So I know the place a lot better.)