If clause type 1 is used if the condition is possible to happen and normally tells about something, that might happen in the future.

If the weather is nice tomorrow, I willl go swimming.
If I eat an apple a day, I won't need a doctor.

 

Erklärung auf Deutsch:

 

Erklärung auf Englisch:

 An "conditional" or "if -clause" has two parts: The "If part" and the "main" part. The "if-part" always shows the condition
that needs to be true for the main clause to become reality: 

If part	Main part
If it rains tomorrow,	we will watch TV.
	we can watch TV.
	watch TV!
If + Personal pronun + Simple Present + rest	Personal Pronoun + will future + rest.                               or can/must/etc.                               or Imparative

You can swap the if-part and the main part:

If it rains tomorrow, we will watch TV.
OR
We will watch TV if it rains tomorrow.

But: If your "If-part" is first, you must put a comma behind it.