***** NOT A TEACHER *****
Hello, LiuJing.
(1) I think that it would be helpful to remember that these are
clauses of result -- not purpose.
(2) I thnk that a comma is necessary if there is any possibility that the
reader will think that it is a clause of purpose rather than result.
(3) Here are some sentences from grammar books that do not use
the comma:
Thelma looked up suddenly to find a cat on the chair.
He stopped smoking only to start again.
(I would say that in speaking, a person would definitely pause after
"smoking.")
He arrived to find the place on fire.
(4) I agree with the other posters that it would be helpful to use a
comma if the word "only" is in the sentence. If the word "only" is
absent, then a comma would not be necessary. In fact, it might be
"wrong."
***** Thank you.