Alternatives to relative clauses

The realtive pronouns can be omitted when they function as the object of a verb or the object of a preposition.

ex: He donated some money to a charity that he greatly admired.
"That” is the object of the verb “admired”, so it can be omitted
He donated some money to a charity he greatly admired.

ex: The man whom she quarrelled with was her ex-boyfriend.
“Whom” is the object of the preposition “with”, so it can be omitted.
The man she quarrelled with was her ex-boyfriend.
However, “whom” cannot be omitted if it is preceded by the preposition: The man with whom she quarrelled was her ex-boyfriend.
ex: She avoids foods that are too spicy.
“That” is the subject of the verb “are”, so it cannot be omitted.
 Smoking is a habit that I detest.
Smoking is a habit I detest.

Love is a game that only two can play .
Love is a game only wo can play.

Donghae is the handsome man whom I told you about.
Donghae is the handsome man I told you about.