"A Conjunction is a word which merely joins together sentences, and sometimes words."
Some Conjunctions and their uses:
Since, as a Conjunction means-
1) From and after the time when; as
Many things have happened since I left college.
I have never seen her since that sad incident happened.
Note: Since, when used as a Conjunction in this sense, should be preceded by a verb in the present perfect tense, and followed by a verb in the simple past tense.
2) Seeing that, in as much as; as
Since you will not work, you shall not eat.
Or is used-
1) To introduce an alternative, as
She must work or starve.
Note: There may be several alternatives, each joining to the preceding one by or, presenting a choice between any two in the series.
You may study medicine or architecture or engineering, or you may enter into business.
2) To introduce an alternative name or synonym; as,
The violin or fiddle has become the main instrument of the modern orchestra.
3) To mean otherwise; as,
We must go now or we will be late.
4) As nearly equivalent to and; as,
The soldiers were not lacking in strength or courage, but they were badly fed.
If is used to mean-
1) On the condition or supposition that; as,
If it rains, I will not go.
2) Admitting that; as,
If I am stupid, I am at least honest.
3) Whether; as,
I asked her if she would help me.
4) Whenever; as,
If you have any doubt, let me know.
If is also used to express wish or surprise; as,
If I only knew!
Than as a Conjunction, follows adjectives and adverbs in the comparative degree; as,
I see you oftener than (I see) her.
While is used to mean-
1) During the time that, as long as; as
While she was sleeping, a thief entered the house.
2) At the same time that; as,
The boys danced while the girls played.
3) Whereas; as
While Tom has no money to spend, Peter has nothing to spend on.
Only as a Conjunction, means except that, but, were it not (that); as,
A very handsome man, only he squints a little.
The evening is pleasant, only rather cold.