Since

As a conjunction since means ‘from the past time when’.

Where have you been since I last saw you?
It is one week since we arrived here.
I have never seen him since I left that city.

Notes

When used as a conjunction in this sense, since is usually preceded by a verb in the present perfect tense, and followed by a verb in the past tense.

The conjunction since can also mean ‘as’.

Since we have no money, we can’t buy anything. (= As we have no money, we can’t buy anything.)

Or

The conjunction or is used to introduce an alternative.

Is it blue or green?
Hurry up, or else we will be late.
You can study hard or you can fail.

Nor

The conjunction nor is still used, though it is not very common. Its most common use is in the correlative pair neither-nor.

She is neither beautiful nor intelligent.
He is neither rich nor wise.

Nor can also be used with other negative expressions.

Peter didn’t turn up, and nor did Harry.

If

The conjunction if means ‘on the condition that’, ‘supposing that’ and similar ideas.

If you want to go, I will you take you.
If it rains, we shall not go.

If can also mean ‘when’ or ‘whenever’.

If I do not wear my spectacles, I get a headache.

If can mean whether.

Do you know if Mr John is at home?

That

The conjunction that can be used to express a reason or cause. It is equivalent to because.

He was annoyed that he was contradicted. (= He was annoyed because he was contradicted.)

That may also express a purpose. It is equivalent to ‘in order that’.

We sow that we may reap.