Can I omit a subject in If-clauses?

I came across a sentence on Cambridge Dictionary. It says

If found guilty, he faces six months in jail.

They omitted "he is" right ? And can I say

If (my computer is) broken , I will fix it.

And can I also use this grammar with an active sentence

If (you) punch him, he gets hurt

Thanks in advance

Solution 1:

This kind of ellipsis only works if the omission consists of the subject and a form of the verb 'to be' and the omitted subject appears in the main clause either as the subject or the object.

If (he is) found guilty, he faces six months in jail. (subject-subject)

If (my computer is) broken, I will fix it. (subject-object)

Of course, the preceding sentence is only intelligible if it is clear from the context what the it is. And some might object that the "if broken" clause is a dangling participle.

The sentence If punch him, he gets hurt does not omit the verb to be. It is both unintelligible and ungrammatical. A grammatical alternative is:

If (he is) punched, he gets angry.

Solution 2:

The first example only works if the context makes it clear who is being talked about.

Fred Bloggs is in court on a charge of [whatever]. If found guilty, he faces six months in jail

The 'If' phrase has to refer to the subject.

My computer is very slow. If broken, it will have to be sent away for repair is grammatical, if not very idiomatic. So, no, your third example doesn't work.