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A sentence: If you think the car is driving too slowly and you are unable to pass, you should pull back.

What's the difference between can't and unable to?
Can I say: "If you think the car is driving too slowly and you can't pass, you should pull back" ?
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Hello Warrener

'Able/unable to' is often used in instructions and descriptions, because it's a neutral phrase with no other emotional connotations.

'Can/can't' on the other hand often has a sense of 'permission', as in your 'Can I say...'

To my ears, for instance, 'and you can't pass' could have a very slight tinge of emotion: as if you've been trying your hardest to pass, dammit, but just can't. (Obviously I'm exaggerating here a little.)

MrP
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Mr P:
'Can/can't' on the other hand often has a sense of 'permission', as in your 'Can I say...'

JTT: I believe, Mr P, that you are conflating two ideas that, while they CAN sometimes be close, are not here. I suggest that it's quite than a stretch to suggest that Warrener is asking permission here. This 'can you' is one of "Is it possible to ...?"

I think that in some language circumstances both work just fine. Though there are nuances where one or the other is excluded, these meanings also share a common middle, if you will, because they share a common meaning.

This probably would be easier to see and explain with a different example though, I must admit, none readily come to mind right now.
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Comments  
you are conflating two ideas

Fair point. 'Can I say' in the original post is not a true 'permissive' 'can'.

MrP
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warrenerCan I say: "If you think the car is driving too slowly and you can't pass, you should pull back" ?
warrenerCan I say: "If you think the car is driving too slowly and you can't pass, you should pull back" ?
AnonymouswarrenerCan I say: "If you think the car is driving too slowly and you can't pass, you should pull back" ?

Yes, you can say that.

AnonymouswarrenerCan I say: "If you think the car is driving too slowly and you can't pass, you should pull back" ?

Yes, you can say that.

CJ