difference between quiet and silent

what is difference between quiet and silent

(Hi, can anyone tell me whether I can use both of these adjectives in sentences like:

     "He was sitting in his room and he was completely quiet."

     "He was sitting in his room and he was completely silent."

     Is there any general difference between them? Thank you very much)

• There is, actually, a difference here. Too often have I seen people use 'silent' where 'quiet' would have been far more appropriate(таарамжтай, зохистой). 
  In your specific case, yes, you may use either. Just know that 'silent' means 'the absence(байхгүй байх) of noise' whereas 'quiet' can mean that there is a little noise, but it was small and did not really bother(яршиг удах) anyone. Too often have I seen people use 'silent' incorrectly:
  
  "He disappeared with a silent pop."
  "'Of course not,' he said silently."
  
  Now, if they were actually silent, there would be absolutely no noise. There would be no 'pop' at all--no noise would be heard if it was silent. Neither can someone speak silently, as speaking is noise. In both of these cases, 'quiet' would have been a far better word to use. 
  
  But, yes, in your case, either is allowed. The question is what you mean. If you want to say that he was silent--perhaps in fear that he would get into trouble if he so much as sighed or because he would be beaten if he spoke--then use silent. If he was simply quiet, out of the way, and not distracting(анхаарал саринуулах, тарчлаах) to anyone, but still making a little bit of noise to amuse(хөгжөөх, цэнгээх) himself, it may be better to use 'quiet'.
  
  I hope I explained that well enough.
  
  Cheers.

• The other day I had to correct one of my students over the meaning of the words “quiet” and “silent”, so that’s what I’d like to write about today.
  The word “quiet” means for there to be very little noise or for someone to not talk very much, but the word “silent” means for there to be no sound at all or for someone to not speak at all. For example:
  It’s very quiet and peaceful at my summer house near the lake. I like to spend my weekends there in July and August.
  A: Do you hear much noise from the traffic in your apartment?
  B: No, I’m not on a main road, so it’s actually pretty quiet.
  I’m trying to study. Could you please be quiet?
  My cousin Beth is a very quiet woman, but she talks a little more when she gets to know someone.
  I went out to the lake last night, and it was completely silent. There wasn’t even the sound of a bird.
  When the singer collapsed on stage, everyone in the audience was shocked and became silent.
  The word “quiet” is more commonly used than “silent” because it’s rare that we hear no sound at all. Also, when we use the word “silent” about a person, it must be used to talk about a specific situation, but we use “quiet” to describe a person’s personality in general.
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