+0
I make a close study of use of "if" 
and I have a questions about grammar in these sentences (I read it in electronic dictionary).

"He asked me if I knew a new address of our university professor"
I don't understand, why did it use a Past Ind. after "if". 
Why I can't say  "He asked me if I know a new address of our university professor"

and
"If he comes tomorrow, send for me".
I don't understand, why did it say "send" in Past Ind. 
It must be a Future or not?
+0
Hello, Linian - and welcome to English Forums.

"He asked me if I knew a new address of our university professor" I don't understand, why did it use a Past Ind. after "if".-- The verb of a dependent clause ('knew') often regresses with the main verb ('asked')

Why can't I say "He asked me if I know a new address of our university professor"-- You can. The dependent verb can remain in present tense if the condition still persists now. But native speakers tend to regress it anyway.

"If he comes tomorrow, send for me". I don't understand, why did it say "send" in Past Ind.- There is no call for past tense! It hasn't happened yet.

It must be a Future or not?-- 'Send' is imperative voice, which applies to present or future only.
Comments  
Thank you very much for your answer and for your speech of welcome. Emotion: smile

Your answer showed me gaps in my knowledge
- I don't know nothing about grammar of 'send'. 
- And I'm inattentive in Syntax (Why can't I say)