Could anyone explain why one is correct and the other is not?

I’ll lend you my umbrella if you don’t have yours - Okay
I’ll lend you my umbrella unless you have yours - Wrong

What's the difference in these sentences? What to use when?

  • 3
    Who said it was wrong? What reasoning did they give?
    – user230
    Commented Apr 10, 2014 at 11:53

1 Answer 1


Actually, your sentence is grammatically correct.

Y will happen unless X means that if X happens, Y will not happen.

So It makes perfect sense to form your sentence as you did grammatically:

I’ll lend you my umbrella unless you have yours.

It means that I am going to lend you my umbrella if you have forgotten yours.

The problem with the sentence is that it is hard to imagine how I will force you to borrow my umbrella, so even if you forgot yours, it would still be up to you to borrow mine or not.

Take this example:

I will beat you unless you give me your lunch money.

Grammatically it is just like your sentence, but this time it makes more sense that I can keep my word.

I will do X, unless you Y only makes sense if X is something that I can actually execute by myself.

You could change your sentence a bit as well:

I will not lend you my umbrella, unless you forgot yours.

This means that X (I will not lend you my umbrella) will not happen if Y happens (you forgot your umbrella).

So now, if you do not have your umbrella, I will lend you mine.

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