We use if to introduce a situation that may be real or possible.
If we miss the bus, we will be late for the meeting.
If she skips her classes, she will fail her exam.
If you want / if you like
If you want you can spend the weekend with us.
You may join us if you like.
If so means if this is true.
Do you want to learn English? If so, you should visit this website. (= If it is true that you want to learn English, you should visit this site.)
The expression if not is used to link a weaker and a stronger word with related meanings.
She is attractive if not beautiful. (= She isn’t perhaps beautiful; she is attractive, nonetheless.)
The laptop was reasonably priced if not exactly cheap.
If necessary / if possible
This is the contracted form of if it is necessary or if it is possible.
I will start in the morning if possible.
We will fight for our rights if necessary.
If is also used to indicate a condition that must exist before something else can happen.
I will help you if you ask me nicely.
She will come if you invite her.
She will be happy if she wins the first prize.
She said that she would buy the house only if they lower the prize.
If is also used to introduce an imaginary situation.
If Martha listened to her doctor and stopped smoking, she wouldn’t fall ill.
Note the use of the past tenses. They suggest that we are talking about an imaginary situation.
If my grandfather were alive today, he would be ninety-five.
We use present tense in the if-clause to talk about real and possible situations.