Question

1. If you meet someone on the street, you should say hello to him.

2. If you meet someone on the street, you should say hello to him or her.

3. If you meet someone on the street, you should say hello to them.

4. If you meet someone on the street, you should say hello to the person.
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Which expressions are grammatical? Can we use 'him' as in Sentence 1? What about the others?

Answer 1

1, 2, and 4 are grammatically correct.

Using "him" as in 1 was the common usage until about the 1960s and 1970s.

You can fix 3 by using "people" rather than "someone."