why I must to use separate ,not singular
This is a sentence.
Two of the children have to sleep in one bed,but the other three have separate ones.that's right.
and this is a wrong sentence,
Two of the children have to sleep in one bed,but the other three have singular ones.
Why？

Question

Answer

Because that's not what singular means. If you want to say that their beds are made for one person, you should say that they are singles, or single beds. If you want to say that the children sleep alone, you should say that they sleep alone, or one to each bed. Or separately, as in your first example.

Singular means 'the only one of its kind', when it doesn't refer to a singular noun.

'She had a singular ability to create chaos and discord.'

Answer

Separate means apart, away from, on it's own.
Example: "The two boys were separated after the fight"

Singular means one, individual.
Example: "I own a single computer, while my friend owns 2"