For example,

How would I know her age without asking?

How can I know her age without asking?

Are these sentences both correct grammatically ? If so, do they have same meaning? If so, which one is more common?


Both are correct, but the meaning is quite different.

How would I know her age without asking?

which is saying you can't determine (in your mind) her age; to know you would have to ask her. It regards an intuitive ability.

How can I know her age without asking?

which is like saying "How can I (find out) know her age without asking? This is more of a technical question; what process can I follow. It is a how-to question.

  • and "How could I know her age without asking?" could go either way, I think. But "How would I know...?" seems like the more likely question and meaning of "How could I know...?" to me. – Mixolydian Mar 5 '19 at 21:14

They are both correct grammatically, but "How would I know?" on its own is by far the more common turn of phrase. "How can I know..." isn't particularly awkward, nor does it break any idiomatic rules, though, as far as I'm aware.

In this situation, I think the meanings between the two sentences are virtually the same. However, "can" is usually used when you're asking if something is possible, but "would," when referring to other people, depends on the person and is asking what actions they will perform if they were to do something.

For instance "How can he do it?" is a question of what possible ways a man has of doing something. "How would he do it?" is a question of what way that man in particular will choose if he is to do the task. Sometimes, "would" is used interchangeably with "can" in colloquial conversation, especially when referring to one's self.


A similar and typical situational formula is

  • I really wouldn't know.

A reaction on a question meaning I really don't know. In my view this is a use of would that is something special and not conform to normal uses of would such as conditional mode, past habit, or as in The door would not open, or in reported speech.

I must admit I have difficulty analysing the use of would in 1 I really wouldn't know or in 2 Why would he do that.

In 2 would expresses astonisment about sth and the idea that sth would not be normal.

In 1 the idea expressed is I have no possiblity to know that./It's impossible for me to know that.

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