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What you are asking about are examples of relative clauses. There are two basic types: defining and non defining. (In this explanation, I sometimes use "RC" to abbreviate "relative clause.)
A relative clause is used to provide more information about a noun.
A defining relative clause modifies the object or subject of the sentence. One may use a relative pronoun optionally when the clause modifies the subject, but the relative pronoun is required when modifying the object. The defining relative clause gives information essential to identifying what is being talked about. Example:
I want to eat at a restaurant. (no RC)
I want to eat at a restaurant which serves steak. (RC defines the type of restaurant)
Notice that there is no comma used. Also, the RC modifies the object, so the relative pronoun (which) must be retained. "That" may almost always used as a relative pronoun, interchangeable with "which," "who," etc, with no change in meaning. When the RC defines the subject, the relative pronoun may be dropped, also with no change in meaning. Example:
The man who is drinking all the Chivas is my boss.
The man that is drinking all the Chivas is my boss.
The man drinking all the Chivas is my boss.
It also makes no difference if the RC is placed at the beginning or end of the sentence:
My boss is the man drinking all the Chivas.
Non-defining relative clauses also provide more information about the subject or object. The differences are that the non-defining clauses' information is not essential to the identification of what is being talked about, it is always separated from the rest of the sentence by commas, parenthesis or dashes, and the relative pronoun (or adverb) must be retained. Example:
My brother, who is a stockbroker, is getting married next month.
If we drop the first comma, this becomes a defining relative clause, because it tells which of my brother is getting married.
This is by no means an exhaustive explanation of relative clauses. Please add.
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