past tense verb
I saw that "use past simple tense when a time period is given " in a book. But It's different when I learned to the other.

ex)
If I had resisted the temptation of instant food, I wouldn't gain weight.

If I had resisted the temptation of instant food, I wouldn't have gained weight last night.

In this case, 'last night' is a time period.
But why does it put 'wouldn't have gained (have pp)', not 'wouldn't gain'?
I'm confused the differences.
please let me know when use have pp and just only verb.

Question

past tense verb
I saw that "use past simple tense when a time period is given " in a book. But It's different when I learned to the other.

ex)
If I had resisted the temptation of instant food, I wouldn't gain weight.

If I had resisted the temptation of instant food, I wouldn't have gained weight last night.

In this case, 'last night' is a time period.
But why does it put 'wouldn't have gained (have pp)', not 'wouldn't gain'?
I'm confused the differences. 
please let me know when use have pp and just only verb.

Answer

Hi Blueclock. Only certain tenses pair when you use the conditional. Simple past is followed by simple present. If I resisted (in general, now and in the future), I would not gain wait. (in general, now and in the future).

Second example: Perfect tense and perfect tense match.  Past perfect with present perfect. This is the rule.  Here we have a direct causality. The reason why you gained weight was the fact that you had eaten food earlier. Remember "Would not gain weight" here is NOT past tense. WOULD is conditional that refers to the present and the future.
Using a conditional in the past- well your are guessing it already- add "have". ->I would have...

Hope this helps a little. Best, Rüdiger