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.