1.The boy who has a limp was in an auto accident. We do not know which boy is meant without further description; therefore, no commas are used.
2.The man, knowing it was late, hurried home. We used comma as the phrase/clause knowing it was late is not necessary to get the meaning of sentence. That means knowing it was late is not restrictive here.
My question is why the sentence #1 is not correct without the clause who has a limp although the sentences have the same categorised subject the man ** and **the boy in the first and second sentence respectively.
Why is it not correct to write by using comma The boy, who has a limp, was in an auto accident? and by not using comma The man knowing it was late hurried home? I'm confused about the defination of restrictive and non-restrictive here.