that

Used as a demonstrative pronoun to refer to a specific person, animal, thing, or idea that is distant from the speaker, or one that has been previously mentioned or is understood. It can also function as a relative pronoun to introduce a clause that provides additional information about a noun or pronoun.

• Look at that! — Referring to something distant or a surprising event.
• Is that your car over there? — Pointing to a specific car.
• The book that I lent you is very interesting. — Introducing a relative clause.

Used before a noun to indicate a specific person, animal, or thing that is distant from the speaker or listener, or one that has been previously mentioned or is understood.

• Can you see that mountain? — Referring to a specific distant mountain.
• That dog barks all night. — Referring to a specific dog.
• I remember that conversation we had last year. — Referring to a specific past conversation.

Used to introduce a subordinate clause, often a noun clause that acts as the subject or object of a verb, or a clause expressing purpose, result, or reason. In many cases, it can be omitted without changing the meaning.

• She believes that honesty is the best policy. — Introducing a noun clause as the object.
• It's a shame that he couldn't come. — Introducing a clause expressing emotion or reason.
• He works hard so that he can achieve his goals. — Introducing a clause of purpose.

Used to indicate degree or extent, typically modifying an adjective or another adverb. It often means "to such an extent" or "so much."

• I didn't realize it was that cold outside. — Indicating the extent of the cold.
• He wasn't that happy about the news. — Indicating the degree of his happiness.
• Why are you driving that fast? — Indicating the extent of speed.