What is the difference between “How do I know?” and “How would I know?”

Question

I am unable to understand the meaning and correct usage of the above two sentences. Can anyone please explain their differences in meaning and usage?

Answer 1

There's no strict rule distinguishing them. Below, I'll explain the differences between what each question suggests and the situations where it's most appropriate.

How do I know?

"How do I know?" usually casts doubt on something that previously was assumed to be true. For example, if someone you've never heard of on the Internet asks you to send them US$1,200, after which they will send you information about a huge inheritance that you are owed, you might say:

How do I know that I'll ever hear from this person again after sending the money?

How do I know I can trust this person?

Or, about almost any answer on ELL:

How do I know that this person is describing English accurately?

It's also used in sophomoric philosophy:

How do I know that this is not all just a dream?

The point is to challenge an assumed proposition by asking for justification. One could answer by providing justification for the proposition in question.

How would I know?

"How would I know?" asks for a method of finding something out if a given proposition is true: something to check for. The word would indicates the conditional mood, suggesting that the question is about a hypothetical situation rather than a current one.

"How would I know if I've driven too far?" "If you come to a big, white barn, then you've definitely driven too far."

However, people also use the conditional mood as a polite "softener". "How do I know?" is quite challenging. It suggests that something has been carelessly or foolishly assumed. So, someone might ask "How would I know?" even about a present situation, in order to avoid sounding challenging.

Also, you might say "How would I know?" about a present situation if the matter in question is simply unknown:

How would I know if this car was stolen?

How would I know if I'm pregnant?

In this sense, you could also say "How can I tell?" or "How could I tell?" The conditional mood suggests that you are temporarily assuming the part after "if" as a hypothesis. For example, "If this car were stolen, how would I know?" or "If this car were stolen, how could I tell?"

With something unknown but not hypothetical, you would more likely say "How can I tell?" or "How do I know?" For example:

How can I tell which version of Safari I'm running?

Since it is known that you are running Safari, there is no hypothesis to base a condition on. But you could also express it as "How could I tell which version…?" (or "How would I know…?"); in this case, the implied hypothesis is "If I were going to find out which version…"

Other uses

These questions have other uses, too. "How would I know … without …?" suggests that you could not know something without the indicated test:

How would we know if they can sing without holding auditions?

How would we know they can sing if we didn't hold auditions?

You can say "How would I know?" as a rhetorical question, to mean that you couldn't possibly know the matter in question, or that there is no answer that could be known:

How would I know how much money she makes?

How would I know if your cat is psychic?

You can't say "How do I know?" in this sense. The conditional mood is necessary because the question is based on an implied hypothesis which you are saying is false: you are implying that you don't know and can't know.

Answer 2

“How do I know?”

Usually used at the beginning of a sentence, this is asking for help determining something. Google suggests the following:

• "How do I know if my iPhone is unlocked?"
• "How do I know what version of Windows I have?"
• "How do I know if I have diabetes?"

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“How would I know?”

at the beginning of a sentence could mean the same thing. Again from Google:

• "How would I know if I was pregnant?"
• "How would I know if my iPhone is unlocked?"

but also seems to get used in such sentences as "How would I know something if I had not been not taught it?"

By itself (that is, not as part of a sentence), it also seems to get used as an equivalent of

“How should I know?”

and that's what I first took it to mean. “How should I know?” is defensive or defiant, and according to the Cambridge Dictionary means "I cannot be expected to know."

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