We often join sentences by putting who, which or that. We use who for people and which for things. We often use that instead of which.

We normally use that, not which after ALL, EVERYTHING, NOTHING, THE ONLY and SUPERLATIVE.

We can leave out object pronouns who(m), which, that.

We can use what to mean THE THING(S) THAT or THE STUFF THAT. What is like a noun + relative pronoun together.

If the relative pronoun can be left out then omit it and use the dash "-" to show that it is not needed.