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I have these sentences which confused me. I get the meaning but quite do not understand the grammar rule.
What is the rule to omit who/ which in these sentences?
A girl who had a sweet voice.
→ A girl had a sweet voice — phoned me this morning
The ball which came through the window hit my head.
→ The ball came through the window — hit my head
The monkey which jumped onto the roof ran along the wall.
→ The monkey jumped onto the roof ran along the wall
As it was said in the comments and in the first answer, the sentences without who/which in your examples are wrong. But, speaking in general, there is really a rule that comes to mind when looking at your examples. This rule regards to the relative (a.k.a adjective) dependent clauses. This rule states:
You can omit the relative pronoun of a relative clause, when this relative pronoun plays a role of an object in this relative clause.
For example, in the sentence
The ball which I bought this morning is quite good.
we can take the relative clause
which I bought this morning
and get rid of the relative pronoun which since it is an object of this relative clause.
The result will be:
The ball I bought this morning is quite good.
The problem with your examples is that relative pronouns in the relative clauses are not objects of these clauses. For example, in the sentence
A girl who had a sweet voice.
the word who is not an object, but a subject of the relative clause. The same applies to the other two examples. So we cannot omit the relative pronouns in these cases.
There is no such rule. The sentences without who/which are wrong.
There is another way to rephrase these sentences: change who/which to that. Maybe this is what you are looking for?
