Student or Learner
Could you please advise me when to use each one? For instance,if I see someone looks so sad, what should I ask him/her? What happens? What happened? or could it be What's happening? or What'd happened?
Jibja, good morning.
(1) If your friend looks sad, you might say: What's the problem?/ What's (What has) happened? / What happened?
(2) You probably should not say, "What's happening?" Here in the United States, that usually is just a friendly greeting meaning "Hello" or "How are you?"