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?
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…"
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.
“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?"
“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."