He is 18 and is studying physics

He is 18 and is studying physics
 

Sometimes people have trouble using the subject pronoun.
When can we avoid using it, in coordinative sentences ? any rules? Is the sentence above correct, because it doesn´t sound correct to me.

The sentence is perfectly correct (apart from physics).

I don't understand the other problem. You could repeat the he:

He is 18 and he is ...., but it is not necessary. You could also say

he is 18 and studying physics.

Is that the question?

yes, it was. What is the rule then? If the subject is the same, it´s not necessary in coordinative sentences? I mean in the second sentence.

I can't think of a rule, but if there is no change of subject, then the pronoun need not be repeated, but it is no error if it is.

He has been working all day and will be hungry when he comes home.:good:
He has been working all day and he will be hungry when he comes home.:good:

I'm tired and want to go home.:good:
I'm tired and I want to go home.:good:

Note that it is the subject in the coordinated clauses that can be omitted, not in any other subordinate clause.

He has been working all day and will be hungry when comes home. :bad::bad:

Edit: note that the verb can be ellipsed if the same:

He is 18 and studying physics
He is 18 and is studying physics
He is 18 and he is studying physics (sounds like emphasis - such as "I told you yesterday - ..)

He works all day and all night (=He works all day and he works all night)

He likes tennis but hates football.
I go tomorrow and come back tomorrow too.

:thumbsup::thumbsup: