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.
Image may be NSFW.
Clik here to view.
This is the main problem, which we must solve.
Image may be NSFW.
Clik here to view.
This is the main problem that we must solve.
Image may be NSFW.
Clik here to view.
She was wearing a coat, that was sky-blue.
