Some basic ideas of the conjunction 'that':

  1. That as a conjunction is commonly used.
  2. That introduces that-clauses.
  3. That-clauses can be the subject or object of a sentence.
  4. That can be left out but not if it begins a clause.
 

The conjunction that introduces that-clauses which are subordinate clauses. A subordinate clause cannot stand on its own as a complete sentence.

  • It is quite likely that we will be late for the firework display.

The part of the above sentence in bold is a that-clause. We cannot use it as a sentence.

  • Wrong: That we will be late for the firework display.

Wrong because it is a subordinate clause, also is called a dependent clause. It must be joined to a main clause to make a complete sentence. The main clause in the above sentence is It is quite likely.

 

A that-clause can come at the beginning of a sentence as the subject or at the end of a sentence as the object.

  • That he got up late does not mean he doesn't have to go to school.
  • He recalled that he had sent the letter sometime last week.
 

We can leave out that wherever it may appear in a sentence but not if it is at the beginning.

  • He said that he would help me with my homework. / He said he would help me with my homework.
  • It is true that her grandmother is one hundred years old. / It is true her grandmother is one hundred years old.
  • That he didn't know anything about it is no excuse. (Not: He didn't know anything about it is no excuse.)
 

More conjunctions:

Each is a pair of words which together are used as a conjunction:

1. providing/provided that: on the condition that; if.

  • We will be there early providing that we can catch the first train.
  • We welcome her to come along provided that she can stay out late.

That in the above two sentences can be omitted.

2. Assuming that: accept as true without proof.

  • I think she will marry the wealthy boss's son even assuming that she doesn't love him.

3. Seeing that: because; accepting the fact that.

  • You may as well join them, seeing that they need another volunteer.