What makes the differences?

Question

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?

Answer 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)

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.

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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.

Answer 2

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.