That is a very common word in both writing and speaking. We use it as a determiner, a demonstrative pronoun and a relative pronoun. We also use it as a conjunction to introduce that-clauses.
That: determiner and pronoun
We use that most commonly to point to a thing or person. We use it with singular nouns. The thing or person is often distant from the speaker and sometimes closer to the listener, or not visible to either the speaker or listener:
Can you pass me that green bowl over there? (determiner)
[pointing to one of a selection of different paint colours]
I quite like that one.
That’s Harold in the white shirt, isn’t it? (pronoun)
We also use that to refer back to a whole clause:
We’re having a few friends round for dinner. Would you like to come?
Why don’t you come at around 8? That’ll give me time to get ready.
Can you tell Kat to hurry up? We’ve got to leave at 11.
I’ve already told herthat.
We use that to refer back to something that has already been spoken or written about:
If he gets that job in London, he’ll be able to visit us more often.
That: relative pronoun
We use that to introduce defining relative clauses. We can use that instead of who, whom or which to refer to people, animals and things. That is more informal than who or which:
She picked up the hairbrush that she had left on the bed.
He was the first director of the National Science Foundation, and he funded science research with an annual budget that grew to 500 million dollars.