I'm a Japanese learner of English. I have some questions about the usage of an auxiliary verb "can."

I think the following two sentences are grammatically correct.

Climbing in winter can be dangerous. Your tomorrow's climbing can't be safe.

However, I'm not sure if the following two sentences are also correct.

Climbing in winter can't be safe. Can climbing in winter be dangerous?

supplementary explanation

In a dictionary, I learned about "can" that expresses possibility (not ability or permission) as follows:

In positive sentences, "can" expresses general possibility, that is, possibility that a thing happens irregularly several times, and "can" isn't able to express specific possibility, that is, possibility that a thing happens specifically only once.

For example, the next sentence is not correct.

It can rain tomorrow.

In contrast, the following sentence is correct.

It may rain tomorrow.

Also, in negative sentences and interrogative sentences, "can" expresses specific possibility.

In the dictionary, there isn't a explanation of whether "can" in negative sentences and interrogative sentences is able to express general possibility. So I post this question. I think possibility that climbing in winter is safe/dangerous is general, that is, it happens irregularly several times.

  • 2
    What are your concerns exactly? May 9, 2021 at 12:12
  • 1
    I trust you realise that the assertion (1) X can be dangerous carries nowhere near the same meaning as (2) X can't be safe. If not, you need to look into that more closely. May 9, 2021 at 14:36
  • Indeed, it's rare to say that something "can't be safe" with the deontic meaning of "can": it implies that "There is no way to may that safe".
    – Colin Fine
    May 9, 2021 at 15:11
  • I made a supplementary explanation. I'd appreciate it if you would read it.
    – Aya
    May 11, 2021 at 4:31

2 Answers 2


Climbing in winter can be dangerous. Your tomorrow's climbing can't be safe.

The second sentence is at least unidiomatic, and arguably wrong. I can't think of a context in which "Your tomorrow's {activity}" would be natural to a fluent speaker. In fact I am not quite sure of the4 intended meaning here. Possible rewrites would include:

  • Climbing in winter can be dangerous. Your planned climb tomorrow would not be safe.
  • Climbing in winter can be dangerous. Your climbing tomorrow cannot (must not) be unsafe.
  • Climbing in winter can be dangerous. I can't imagine that it would be safe fro you to climb tomorrow.

The two sentences:

Climbing in winter can't be safe. Can climbing in winter be dangerous?

...are both grammatically valid and each sounds natural. They don't fit well in the same statement because their meanings are opposed, of course.

The sentence:

It can rain tomorrow.

...is correct if it is expressing physical possibility. For example:

The monsoon season is almost here. It can rain tomorrow.

This would be more naturally expressed as "it could rain tomorrow", but I think the form with "can" will be used in some cases. Certainly one might write:

The rainy season usually arrives in July, but it can rain in June


All of your example sentences are grammatically correct, apart from

"Your tomorrow's climbing can't be safe."

This is standard usage of "can" just with a negative attached, but I would use "Your climbing tomorrow can't be safe." as tomorrow is abstract and you cannot possess it. Your understanding of "can" is perfect however.

  • 1
    "The climb you plan for tomorrow can't be done safely." May 9, 2021 at 16:51
  • 2
    i read "can't be safe" as "Blimey! that can't be safe!" i.e. hyperbole May 9, 2021 at 16:54
  • For example, is the following sentences also correct? "For old people, climbing in winter can't be safe." "For old people, can climbing in winter be dangerous?"
    – Aya
    May 14, 2021 at 6:18
  • 1
    "Tomorrow" can be used in a possessive form: "Tomorrow's weather will be sunny" but "Your tomorrow's climbing" is defiantly not idiomatic, indeed I would label it incorrect. @Aya Oct 25, 2021 at 21:38

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