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Are there any differences between these two expressions?

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4 Answers

up vote 2 down vote accepted

When talking about physical or other external (im)possibility in the future, they are synonymous:

   "You can't get in to the building tomorrow"
   "You are not going to be able to get into the building tomorrow"

But if the issue is of permission, you would not normally say

"You are not going to be able to go tomorrow" 

unless perhaps you mean that somebody who has authority to give you permission has not yet made a decision but you think that tomorrow they will forbid you to go.

Since "you are not going to be able to" has a future feel to it, it also cannot be used for a an impossibility or prohibition right now:

"You can't do that!"
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Yes, using "you can't..." doesn't imply any reason for what's keeping you from doing something. It might be something that you are clearly able to do, but should avoid for some other reason.

The expression "you can't..." can also be used as a request to keep someone from doing something, in that case the person is always able to do that action, or there would be no point in the request.

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I don't see any relevance to either of these suggestions, unless you mean the distinction between "possible" and "permitted" that I have made in my answer. –  Colin Fine Feb 28 '11 at 18:04
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The two sentences have a slightly different meaning because the second sentence contains going to.

She can't speak Italian.
She is not going to be able to speak Italian before tomorrow.

While the first sentence is equivalent to "she is not able to speak Italian", the second sentence is roughly equivalent to "she will not be able to learn Italian and speak it before tomorrow."

The NOAD (third edition) reports the following note about can:

Is there any difference between can and may when used to request or express permission, as in "may I ask you a few questions?" or "can I ask you a few questions?" Many people feel that can should be reserved for expressions denoting capability, as in "can you swim?", rather than for those relating to permission. May is, generally speaking, a politer and more formal way of asking for something, and is the better choice in more formal contexts.

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I would say that most commonly, "You're not going to be able to" is used in response to a stated intention to engage in a future action:

"I think I'll go to the fair tomorrow."

"You're not going to be able to do that, you already promised Mom you'd spend all day re-grouting her kitchen."

"You can't" is often used in response to an action that has just been taken or is about to be taken, as a precursor to an explanation of why that action is against the rules.

"I'll move my bishop here."

"You can't do that, bishops can only move diagonally."

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