Student or Learner
1) We light a candle. But what is the opposite? is it "put out"? Is this the best verb?
2) Is it correct to say "a candle burns until it puts out"?
We often blow out a candleor blow a candle out. We can also snuff out a candle, snuff a candle out, or simply snuff a candle. If we are being formal, we can extinguish it.
Is there a special expression in English for putting out a candle with your thumb and index finger moistened with saliva?
Not as far as I know. That's including in 'snuffing (out)'.
ps. (A few moments later.) I think I remember my grandmother 'pinching out' candles, but I am not sure about that.