I won’t and I’ll not are both short forms of I will not. Both are used in English.
Are there any situations where one is preferred over other?
Learn English – “I won’t” vs. “I’ll not”
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Related Solutions
Just and simply have some overlapping meanings, but definitely do not overlap in all meanings. In your example, it is a case where they have essentially the same meaning.
However, just (the adverb) can also mean "only now". For example:
How long have you been here? — I just got here. (=I arrived right now.)
Simply cannot be used in this instance.
(Note: I assume that we are only talking about just as an adverb; just can be used as an adjective too, meaning "right" or "fair". I don't think you are concerned about that one, because it is clearly totally different in usage.)
Google N-gram Viewer suggests that unexplainable is a recent invention (at least in written form):
Best Answer
The following is from my own (and, I trust, a general British) perspective. This one would be very difficult to provide supporting references for.
I won't is the choice when an emphasised statement or retort is given:
"You must do two hours unpaid overtime tonight."
"I won't! / I will not!" (not "I'll not!"; won't here connotes strong will)
"There's no other way – you'll have to drive the truck through the minefield."
"I won't stand a chance!" (preferred) ("I'll not stand a chance!" sounds weaker)
"I won't inform the police that the man who ran on to the pitch and thumped the player was your brother." (won't stresses my – perhaps concessive – non-involvement; I'll not would be an unemphasised statement of intent)
I'll not is also short for I shall not, but the will / shall complexities seem to be very unresolved and argued over.