As you know , the English teachers in China are always teaching grammars in Chinese, i think it doesn't help for English learners, why don't we conquer it in English? From now on, i will teach u the most difficult grammar in Chine:

Adjective clauses (relative clauses) are like "sentences
inside sentences." The "job" of adjective clauses is to
modify (describe, identify, make specific) the noun
phrases that they follow. In their full forms, adjective
clauses have several parts: a relative pronoun (or, in
some cases, another kind of connecting word), a subject,
and a predicate (a verb and, often, other types of
words which follow it).

In adjective clauses, the relative pronoun is a kind of
connecting word: it joins the information in the clause
to the noun phrase that it follows. Without the adjective
clause, the meaning of the modified noun phrase (and
of the sentence) is unclear and incomplete.

Examples (full forms):

I know a person who / that can help you.
I know a person who(m) / that you can help.
I know a person whose advice I can trust.
I know a person to whom I can refer you. /
I know a person who(m) / that I can refer you to.

I want a car that / which gets good gas mileage.
I can't afford the car that / which I really want.