Since as a conjunction
As a conjunction since can refer to time.
- Many things have happened since I left Mumbai.
- I have never seen them since that unfortunate event happened.
When since is used as a conjunction, it should be preceded by a present perfect tense and followed by a verb in the past indefinite tense.
Or
The conjunction ‘or’ is used to introduce an alternative.
- You must work or starve.
- You may take this coat or that one.
The conjunction ‘or’ is nearly equivalent to ‘and’.
- Our troops were not wanting in strength or courage, but they were badly fed.
If
If can mean ‘on the condition that’.
- If he is here, we will meet him.
- If he wants to succeed, he has to work hard.
If can mean ‘whether’.
- He asked me if I would help him.
If can also be used to express a wish or surprise.
- If only I knew more people.
That
That is a demonstrative pronoun. It is also used as a conjunction. As a conjunction, that has retained much of its force as a relative pronoun.
Hence the sentence ‘I heard that you were looking for a job’ may be transposed into ‘You were looking for a job: I heard that.’
The conjunction that can be used as an equivalent to because.
- He was annoyed that he was contradicted. (= He was annoyed because he was contradicted.)
That can also express a purpose.
- We eat that we may live.
- We sow that we may reap.
- He kept quiet that the argument would stop.