1) Unless: We usually use "unless" in order to mean "if not". If we compare the sentence with "if not" and "unless" we will see that "NOT" is not used in the sentences with "unless".
If you don't come, I won't invite you again.
Unless you come, I won't invite you again.
Unless you leave home at once, you will be late for work.
You can't pass the Yds exam unless you memorize a lot of words.
• In the unless clause , a negative statement can also be used. In this case the meaning is emphasized.
Don't go out in the midday sun unless you really can't avoid it.
(=Don't go out in the midday sun if you can avoid it.)
• "Otherwise" which is an adverb also means "if not".
The students obey all the rules otherwise they will be punished.
2) provided/providing that ,as long as/so long as, only if
These structures are all in the meaning of IF. However, the meaning is more stressed.
You can pass the exam only if you study hard.
You can pass the exam provided that you study hard You can pass the exam providing that you study hard.
You can pass the exam as long as you study hard.
You can pass the exam so long as you study hard.
We may use all these expressions in the beginning of the statement but the main clause is inverted in the sentence with "only if".
Providing that you study hard, you can pass the exam.
Only if you study hard can you pass the exam.
3) Even if:
We use "even if" when we want to give the idea that a particular condition does not matter, the result won't change.
We have decided to go on a picnic even if it rains. I won't forgive him even if he apologizes to me Seyfeddin would have gone to the mosque even if his father hadn't allowed.
4) Whether or not: We use "whether or not" (Not "if or not")in order to introduce alternative conditionals. We use this item in order to express the idea that neither this condition nor that condition matters.
I will go to the party whether they invite me or not.
(=Even if they don't invite me to the party, I will go.)
5) Suppose that ; Supposing that ; On condition that ; What if
Suppose(that) you won the big prize, what would you do?
Supposing (that) we miss the train, what shall we do?
You can take my dictionary on condition that you bring it back by Friday.
What if your pay check doesn't arrive in time?
(=What will happen if your pay check doesn't arrive in time?
What if he didn't come tomorrow?
(=What would happen if he didn't come tomorrow?)
What if you hadn't learnt the news?
(=What would have happened if you hadn't learned the news?)
6) In case :
It expresses a sort of precaution.
"In case and if"are completely different.
I will take my umbrella in case it rains.
I took my umbrella in case it rained.
I will go to my mother in case she needs help.(...ihtiyacı olur diye...)
I will go to my mother if she needs help.(...ihtiyacı olursa...)