Choosing a relative pronoun

Choose carefully between that, which, and who.

As a general rule, use that when a relative clause is restrictive (without a comma), which when a relative clause is non-restrictive (with a comma), and who in either case whenever the antecedent is a person.

This is the main problem, which we must solve.

This is the main problem that we must solve.

She was wearing a coat, that was sky-blue.

She was wearing a coat, which was sky-blue.

I know a man that owns twenty horses.

I know a man who owns twenty horses.