Some conjunctions and their uses

Since

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

• Where have you been since I last saw you?
• It is just a week since we arrived here.
• I have never seen him since that unfortunate event happened.

The conjunction since may 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 green or blue?
• You can have tea or coffee.

Sometimes or is used as an equivalent to and.

• The troops were not wanting in strength or courage, but they were badly fed. (= The troops were not wanting in strength and courage…)

If

The conjunction if means ‘on condition that'; ‘supposing that’.

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

If can mean ‘when’ or ‘whenever’.

• If I don’t wear my spectacles, I get a headache.

If is also used to express wish or surprise in the structure if only.

• If only I had known that. (Emphasizing one’s regret that one did not know it.)

That

That is simply a connector. It is used to express a reason or cause.

• His manners are so bad that nobody invites him to a party. (= Nobody invites him to a party because his manners are so bad.)
• Bring it to the light so that I can see it better.