We use the word that in many different ways in English. Let’s look at the proper usage together.
1.) That the applicant didn’t arrive on time for the interview was all we needed to know.
In this case, that introduced the subject of a sentence.
2.) That receipt on the desk needs to be filed. That needs to be filed.
In the first sentence, that serves as a demonstrative pronoun. In the second sentence, it is a determiner.
3.) The secretary brought the documents that the boss wanted.
Relative pronouns connect a relative (or dependent) clause to an independent clause. That is one of the relative pronouns that we can use to introduce the relative clause, the clause that gives us more information.
That is a bit less formal that which, and does not take a comma before it.
Correct: The desk that is in the corner will be yours.
Correct: The desk, which is in the corner, will be yours.
Notice that that and which refer to things. If you want to talk about a person, use the relative pronoun who.
Correct: The accountant who is new here is doing a great job.
Incorrect: The accountant which is new here is doing a great job.
Informal: The accountant that is new here is doing a great job.
4.) The client said that she would arrive late.
That indicates that the clause that follows is the object of the verb.
5.) The worker was angry that he didn’t get a raise.
That starts a clause that will give us more information about a noun or an adjective. Here, is tells us why was the a worker angry.
6.) We have moved to a new location so that we can serve our customers better.
That can be part of a compound conjunction, a phrase that works as a connector. Some examples of compound conjunctions are as soon as, as well as, and as if.
7.) The manager implied that the staff had no choice but to work all weekend.
We use that as part of the sentence structure in report speech.
8.) I am happy to accept the offer (that) you have made.
When we refer to an object in a relative clause, that is optional.
We can omit relative pronouns when they are the object of the verb that follows them.
She is the manager (who/that) we talked about yesterday.
We can’t omit relative pronouns when they are the subject of the verb.
She is the manager who/that yelled at the junior staff.