That is one of the commonest words in English. It is a relative pronoun. As a relative pronoun that can refer to both people and things.
- The letter that came this morning is from my father.
Here the relative pronoun that refers to a thing – the letter. In this case it is possible to replace that with which.
- The letter which came this morning is from my father.
Another example is given below.
- Give help to anyone that needs it.
Here the relative pronoun that refers to people. In this case it is possible to replace that with who.
- Give help to anyone who needs it.
That can also be used as a conjunction. It is the most common conjunction used to introduce a noun clause.
- She told me that she didn’t want to come.
Here the conjunction that introduces the noun clause she didn’t want to come. Together they act as the object of the verb told.
- I will see that dinner is ready for you when you get home tonight.
Here the noun clause ‘that dinner is ready for you’ is the object of the verb see.
The conjunction that can be dropped when it is used to introduce a noun clause.
- She told me that she was interested in the offer. OR She told me she was interested in the offer.
- Now I realize that I was wrong. OR Now I realize I was wrong.
That can also introduce adverb clauses of reason and purpose.
- Her behavior is so bad that nobody likes her.
- Bring it closer so that I can see it clearly.
- You may go on condition that you will be back before midnight.
That is also used in exclamations.
- Oh, that I could walk again! (= How I wish it were possible for me to walk again!)