i am confused in using *as long as* and *unless*. please give me some direction.

"I will go if it is free." (If it is free, I will go.) In this sentence we can change if to as long as and it keeps the same meaning: "I will go as long as it is free." (As long as it is free, I will go)

what is the diffrence between if and as long as?

"If it isn't free, I won't go." (I won't go if it isn't free.) Here we can use unless to keep the same meaning: "Unless it is free, I won't go." (I won't go unless it is free.) Unless means except if or only if.