If it rains we shall postpone the picnic.

If it rains is a dependent clause
we shall postpone the picnic is independent. I say to put comma between rains and we, Correct?

I wouldn't put a comma there. It's so short, a comma isn't needed,

I know to use a comma when a sentence is introduced by a dependent (subordinate) clause.

Is a comma needed with two independent (subordinate) clause
Example--The classes will be held unless the schedule has been changed.

Or is, unless the schedule has been changed-----is a dependent clause.

No. A comma is not needed there.

Yes, you are correct. In this sentence, "If it rains" is a dependent clause because it cannot stand alone as a complete thought. The independent clause "we shall postpone the picnic" expresses a complete thought and can stand alone as a sentence. When combining a dependent clause with an independent clause, it is common to use a comma to separate them. Therefore, the correct way to punctuate the sentence is: "If it rains, we shall postpone the picnic."