Incorrect: One should not waste his time.
Correct: One should not waste one’s time.
Correct: A man / boy / girl  / woman should not waste his / her time.

The indefinite pronoun one when used in a sentence should be used throughout.

Incorrect: ‘Have you a pen?’ ‘I haven’t got.’
Correct: ‘Have you a pen?’ ‘I haven’t got one.’

Got is a transitive verb. It must have an expressed object.

Incorrect: He enjoyed during the holidays.
Correct: He enjoyed himself during the holidays.

Enjoy is a transitive verb. It should be followed by an expressed object.

Incorrect: I asked for his book, but he did not lend me.
Correct: I asked for his book, but he did not lend it to me.

Some transitive verbs (e.g. give, lend etc.) should have two expressed objects.

Incorrect: The boy who does best he will get a prize.
Correct: The boy who does best will get a prize.

Incorrect: Whoever works hard he will win.
Correct: Whoever works hard will win.

Incorrect: The girl who answered the phone she was polite.
Correct: The girl who answered the phone was polite.

Incorrect: I shall see the brakes whether they work well.
Correct: I shall see whether the brakes work well.

Incorrect: He has got a new girlfriend who she works in a bar.
Correct: He has got a new girlfriend who works in a bar.

One subject or object in a clause is enough. We don’t have to use a pronoun when the noun itself is present in the clause.