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What makes the differences?

  1. You can walk home if you get your crutch. (Can should be replaced with “will be able to” otherwise it sounds like the other person will be permitted to walk him if he gets a crutch)

  2. You can open the door if you press the button. (Can sounds fine)

  3. You can stop the war if you kill him. (Can sounds fine)

What makes the differences?

Is it because example 2 and 3 are abilities that the speakers already have and can use as long as they complete the conditions instead of the ability they will only gain in the future?

E.g: In the first sentence, the other person currently doesn’t have the ability to walk while in sentence 2 and 3 they already have the ability to open the door & stop the war; all they need is to complete the conditions in the if clause to utilize their existing capacities.

Is this a valid analysis?

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    For the absolute most proper usage, many sources will tell you that you mustn't use "can" when you mean permission; you must use "may." (Though common practice "breaks" this rule often.) I'm not sure why #1 sounds wrong to you; I see no problem with it. This "may"/"can" rule would eliminate any confusion if it were observed, but even if not, context or even tone of voice would make it clear. Mar 20 at 17:19
  • If some source told you that there's a problem with #1, please edit to tell more about that. Mar 20 at 17:20
  • Note: "It's too bad you left your crutch at home. You could walk home if you had it." In this case you can't use "can." And yes, because we're talking about an ability that you don't have and can't get right now, just imagining it. Is this maybe what you're thinking of? Mar 20 at 17:22
  • The problem is I want to talk about the opportunities or abilities to do something instead of the permission to do so. “You can walk home if you get a crutch” would only be interpreted as a command (you will be permitted to walk home if you get your crutch) while what I really want to say is that “you will gain the ability to walk home if you get your crutch” Mar 20 at 17:27
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    @MichaelHarvey: Why stop there? if you can get some bacon you can have bacon and eggs - if you can get some eggs as well, and if chef says you may use his kitchen. Mar 20 at 19:56

2 Answers 2

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You can walk home if you get your crutch. (Can should be replaced with “will be able to” otherwise it sounds like the other person will be permitted to walk him if he gets a crutch)

This is the disagreement. There isn't actually any need to change this sentence. "Can" is often used both for "able to" uses and "permission to" uses:

  • If you finish your vegetables, you can have dessert.
  • If you look through the telescope, you can see Jupiter.

Context and/or tone help us tell the difference. Eating dessert often requires permission, while seeing Jupiter doesn't. The first example is probably saying "I will let you have dessert only if you eat your vegetables," while the second is probably saying "You will be able to see Jupiter only if you use a telescope." One could maybe invent unlikely scenarios that would have the opposite meaning ("Hey, you can have dessert any time you want, but look, there's no room on your plate! The vegetables are in the way"), but if we were in this unlikely situation, we would know it.

As is often the case, if we have a single isolated sentence without its real context, we can't be 100% sure of its meaning. But the example about walking home with a crutch is more similar to the telescope/Jupiter example than the vegetables/dessert. A crutch is a tool needed for walking. There are few situations in which someone is held against their will and prevented from going home, and of those situations, it's hard to imagine one in which getting a crutch changes the permissions.


Note, as mentioned in the comments, if permission is intended, then "may" can be used with no ambiguity. Also: you're right that "abilities you can gain" is important. "Can" can only be used for an ability you have or can actually gain. If it's something you can't do and can't become able to do, you can't use "can" and should use "could" or something similar: "If you had wings, you could fly."

(To those who would insist that you must not use "can" for "permission" uses, Merriam Webster says:

Can and may are most frequently interchangeable in uses denoting possibility; because the possibility of one's doing something may depend on another's acquiescence, they have also become interchangeable in the sense denoting permission. The use of can to ask or grant permission has been common since the 19th century and is well established, although some commentators feel may is more appropriate in formal contexts.

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  • What I am saying is that in terms of the “able to” meaning. “You can walk home if you get a crutch” should be revised into “You will be able to walk home if you get crutch.” while examples 2 and 3 sound fine with “can.” I am not talking about the permission meaning at all. Mar 21 at 0:15
  • My reasoning is that example 1 emphasizes the ability will be gained after the completion of the condition while examples 2 and 3 sound like the doer already have the abilities, but they need certain conditions to be done in order to use their existing abilities. Mar 21 at 0:18
  • Your Jupiter example is good. Compare these instead: If you look through the telescope, you can see Jupiter vs If you find the key, you can escape. See? The first one means that the other person already has the ability to see something but they have to look through the telescope to well utilize this capability; by contrast, the person doesn’t have the ability to escape until they find the key, and therefore the second one requires “will be able to” instead. Mar 21 at 0:21
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    @ChienTeLu I understand your premise, but I'm afraid it's wrong. It's ok to use "can" even in these cases. There is not a distinction between "things you can do now vs things you can get the ability to do," as there is with "things you can't do, but could imagine." Now, if you want to say that you can replace "can" with "will be able to," sure, that's true. But not that you need to. Mar 21 at 1:45
  • Whether may can be used without ambiguity just depends on context. "You may eat cheese tomorrow" could mean either that you may happen to eat cheese tomorrow, or that you are allowed to eat cheese tomorrow. Mar 22 at 18:05
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  1. You're not completely wrong in your assessment of the first sentence, but it's certainly not a must that the replacement be made. I agree that "will be able to" sounds better, but there's nothing wrong with it as it is. Context and how it is said would play a big part on how it is understood.

  2. Seeing as we are being critical of these sentences - this doesn't sound 'fine' to me; in fact, it sounds far worse that the first sentence. A native speaker might well say this extemporaneously, but in writing, when you think carefully about your words, you spot mistakes. Saying you can do this if you do that suggests a sequence of events - doing the first will mean you can do the other. Maybe it's because there's no context here, but does the button open the door? If so, I would say "you can open the door by pressing the button". If the button just releases a locked door then maybe this is okay, but it shows context is king.

  3. Again, no context given - but taking what I said in the previous paragraph about one thing leading to another - if you're suggesting that the act of killing one person will end the war, there is no sequence of actions. I would say "you can end the war by killing him". Again, no context given, but it seems odd to me that anyone would say kill this one dude, and then you are able to go and stop an entire war in a completely separate act.

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  • How about: You can breathe if you take off the mask. This one definitely requires “will be able to” Mar 21 at 0:02
  • @ChienTeLu yep.
    – Astralbee
    Mar 21 at 0:03
  • It’s basically the same structure as the first sentence, so I don’t know why you think this one requires “will be able to” while “You can walk home if you get a crutch” doesn’t. Mar 21 at 0:09
  • @ChienTeLu Sorry, I don't see a problem with "can" in this example either. (And while I acknowledge Astralbee's preference for "you can do X by doing Y," I'm not sure it bothers me as much or that I would insist on it. Mar 21 at 1:48
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    If you look at @ChienTeLu activity page you'll see that the OP in 4.9 years has cast three upvotes, and never accepted an answer. That's their prerogative, there are no rules saying users must vote on posts but without votes being cast we would not have a "living" Q&A community. The OP will comment about alternative versions but will never admit an answer was helpful. I know we answer questions to help future visitors but the OP's questions are very specific about certain usages that no one else appears to share.
    – Mari-Lou A
    Mar 21 at 10:25

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