Eli5 – How would you explain when to use “I’ll” vs “I will” to someone who isn’t a native speaker?


For example why does it sound unnatural to say “I’ll not use this for profit.”?

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It’s formality Vs informality along with certain phonetic combinations being less common than others, AFAIK

Because the correct use of “I’ll not” is “I wont”. That’s why it sounds unnatural, grammatically speaking.

ETA: I teach English in Europe and I tell my students that they can use those contractions while speaking, but not while writing, because it’s harder to convey tone that way. They should also never use it in a formal letter.

In English, contractions aren’t mandatory per se. The only difference between contracted and uncontracted is formality, the latter being more formal than the former

That phrase sounds unnatural because it’s common to contract “I will not” to “I won’t” instead of “I’ll not”.

You also want to avoid having “I’ll” be a phrase all on its own. Something like “If she doesn’t, I’ll” sounds awkward. You have to have more to say about what it is you will be doing.

Make sure they understand that “I will,” as a complete sentence, should never be contracted to “I’ll.”

Otherwise, the nuance between “I’ll” and “I will” is so minor as to be barely worthy of mention. Neither is incorrect.