Is this sentence complex compound or simple? The question is, when water evaporates from the sea it becomes fresh water
It has one independent clause:
it becomes fresh water
and one dependent clause:
when water evaporates from the sea
What kind of sentence is it?
There's a third, isn't there?
Oops. My bad. I misread. Ms. Sue is quite correct.
I read a third clause at first, too.
To determine whether the sentence is complex, compound, or simple, we need to break it down into its constituent parts.
"The question is" is an introductory phrase that does not affect the type of sentence structure.
The main clause of the sentence is "when water evaporates from the sea, it becomes fresh water." This clause contains a subject ("water") and a verb ("evaporates" and "becomes") and expresses a complete thought.
Since there is only one main clause and no additional clauses or conjunctions, we can classify this sentence as a simple sentence.