the future tense of can
If you have a free calendar tomorrow, and the ability to drive to a place, you can interpret 'go tomorrow' as a thing that you are currently able to do, thus present tense: I can go tomorrow. However, if you are talking about the situation changing, say, your car breaks down this afternoon, then in that hypothetical, which would happen in the future, you would need to use the future tense for the thing you are discussing being able to do: if my car breaks down this afternoon, I won't be able to go tomorrow.
great example, DemirKhomus! this is the type of sentence that actually belongs in the textbook. definitely a situation where won't be able to cannot very well be replaced by can't or won't. wish I had thought of it!
Here's another, based on the same concept:
If my brother doesn't get the birthday present he asked for, I won't be able to take all of his complaining.
Time words aren't even important. It's the predictive nature of the statement that calls for won't be able to because it's taking place in the hypothetical future.
lilyyang, your question is actually more complicated than it seems.
"Can" is a modal verb in English. https://en.wikipedia.org/wiki/Modal_verb (The second sentence in this article has been messed up, so just ignore it.) There are any number of ways of making the same request, namely using 'could you?' or 'would you?'
"Can you go instead of me tomorrow?" sounds a little awkward to my ear, but is perfectly good English. "Will you be able to go instead of me tomorrow?" doesn't sound any better or worse.
As Mycroft suggests, you might offer an explanation first, followed by: Could you go instead? For example:
I know it's my turn to go to the store tomorrow, but I'll be working late. Could you go instead?
Time words like 'tomorrow' don't affect the use of 'can.' You could just as easily say, "I can't go tomorrow" as "I won't be able to go tomorrow." They are equally acceptable.
The problem is that it is not grammatically acceptable in English to use a form of 'will' and 'can' together in a sentence. It is not acceptable to say, "I will can go tomorrow" or "I won't can go tomorrow" (won't = will not). Instead, you need to say "I'll be able to go tomorrow" or "I won't be able to go tomorrow."
It's not exactly correct to say that the future tense of "can" is "will be able to." The real issue is that "will can" is not a generally acceptable form of grammar in English. Instead you need to say "will be able to" or "won't be able to."
Thank you so much for clarifying that.
lilyyang, can you describe the situation in which you wanted to say, "Can you go instead of me tomorrow?"
For example, if you were scheduled for an important meeting, but for some reason or other you were unable to go as scheduled and needed a colleague to go to the meeting in place of you, you might ask, "Could you go in my place?" To me that sounds better than, "Can you go instead of me?"
brucenator, thanks again for the kind help. I just made an example of "can" by myself at that time, there's nothing to do with any situations.
I saw a sentence pattern "I'm afraid I won't be able to help you" from the textbook "Side by Side" 2, page 98, and wondering if I can use "can" in the situation below.
A: I'm afraid I won't be able to help you move to your new apartment tomorrow.
B: You won't? Why not?
A: I've got to take my son to the doctor.
B: Don't worry about it! I'm sure I'll be able to move to my new apartment by myself.
As a beginner, I will use "can" in that situation, not "won't be able to".
Actually, I think I use "can" in most situations. It's easier for me to translate what I try to say, I guess.
This would work just as well. Both "can" and "will be able to" are equally correct. Present tense (as well as present continuous - I'm eating at Denny's tomorrow etc.) is frequently used as future tense.
A: I'm afraid I can't help you move to your new apartment tomorrow.
B: You can't? Why not?
A: I've got (I have) to take my son to the doctor.
B: Don't worry about it! I'm sure I can move to my new apartment by myself (on my own).
Exactly right. I would only point out that "won't be able to" is less direct and can soften things a little bit. (Something grammar doesn't teach you.) For example, a friend has invited you to a party. If you say, "I'm afraid I won't be able to make it," it sounds a little softer, more regretful, more apologetic than saying, "I'm afraid I can't make it." Whenever you want to go for a softer, less direct approach, "I won't be able to" is almost always a better choice than "I can't."
Thanks a million. I didn't know there was a subtle difference between them.
I learned something new today. Really appreciate it.
Not to confuse things further (and please ignore this right now if it does), but...
It also depends on the context! For example,
I have to start working on my project right now, otherwise I won't be able to make it in time for the Science Fair.
"Otherwise I can't make it in time" wouldn't sound right in this context. But why is that?
Upon closer inspection, it's because "will not make it in time" is the predominant idea in this context (not being able to is kind of redundant). You could just as easily say: "Otherwise I won't make it in time for the Science Fair."
But grammatically it follows the same rule: It is not acceptable to say, "I won't can make it in time," but rather "I won't be able to make it in time."
Yes, that works just fine and it sounds a little better than "Will you be able to go instead of me tomorrow?". Alternates:
"I can't (don't want to) go tomorrow, can you go instead?"
"I can't go tomorrow, are you able to go instead?"
"I can't go tomorrow, will you be able to go instead?"