# Usage of a or one, when to use what

It's a simple but stupid question, I don't really know when to use "a" and "one" like. One chance or a chance. One way or a way. What's the difference or there isn't any?

That's not a stupid question, because there are cases where either could be correct but the usage depends on context, intent, and emphasis.

"One" can indicate that it's solitary and final.

There is (only) one solution to this problem. Solve it, or we all die!

There is (only) one way out of the maze. Find it, or we all die!

You have (only) one chance to save the day. Do it now, or we all die!

OR, "one" could indicate that the speaker doesn't think it's the best of all available options ...

True, that is one way to solve the problem, but if we don't want to all die, this might be a better option ...

This is one way to get off the mountain, but we should keep searching for a safer route.

"A" can indicate that there are other known/likely alternatives:

This is a solution to the problem - it's not the cheapest or most elegant option, but it works!

Here is a way down the mountain, or we could ski over to that other one instead.

This is a chance to talk to that girl you like, but you'll have another opportunity next week too.

You can also use "a" to refer to the one option remaining if you thought you were out of options.

There's still a chance to save the day! I didn't see that coming!

Here's a secret passage leading out of the cave we were trapped in! We're saved!

If you're simply asking for an object, using "one" or "a" depends whether you care about how many objects you receive.

Please give me a hammer (because of course I only need 1. If I needed more I would ask for a specific number).

Please give me one (and only one) nail.

The obvious difference is that of implying future intention - If I give you one chance I'm implying you will get only one chance. If I give you a chance, it doesn't provide the same restriction of intent in that I could give you another chance at a later stage.

• On the other hand "There's one way [of solving this problem]" could mean a new way that the speaker has just thought of, rather than the only way. It's not a stupid question at all if you speak a language where the indefinite article is the same as the number one. It's really difficult to formulate a rule for the distinction in English.
– Kate Bunting
Sep 27, 2017 at 7:47

One is a quantifier. Before a noun it specifies how many of that noun the speaker is talking about:

I know one way to get to Hong Kong, and that way is to fly.

There may be more than one way to get to Hong Kong; indeed the speaker may know more than one way; but the speaker is talking about a certain quantity of ways to get to Hong Kong, and that quantity is one. Not two, not three, not 101 but one.

A/an is the indefinite pronoun. It evolved from The Old English word meaning one, so it’s no wonder they often are very close in meaning. You use the indefinite article to make an indefinite noun phrase, as in

I know a way to get to Hong Kong, and that way is to fly.

The simplest thing to say is that the speaker is not giving us a quantity. Yes, a way is equivalent in number to one way (Note again that a/an evolved from one) but a quantifier and the indefinite article do not mean the same as far as the speaker’s assumption or intention. An indefinite noun phrase is most often used when the speaker does not assume that his hearer can identify which thing he is talking about or does not care whether the hearer can identify it.

Let’s try a different sentence to make that clearer:

A: I know a way to get to Hong Kong.

B: What is it?

A: It is to pay this 77-year-old ex-monk named Chan to take us at night by boat.

When A says a way to get to Hong Kong, he does not assume B knows which way that A is talking about. And that assumption is likely correct, since B has never heard of this way before.

Another example. There is a guy (called Joe) who wants a hammer. There are seven hammers on the shelf top.

Bring me one hammer.

The speaker, Joe, is asking for the quantity of one when it comes to number of hammers. One hammer, not two or three or seven. One is all I want.

But if Joe says

Bring me a hammer.

Grammatically, this means he doesn’t care which hammer he gets. It can be any of the seven hammers on the shelf. It is similar to saying any hammer. It’s true that a hammer in this context is equivalent to the quantity of one hammer, but that is not what a hammer means, and ultimately the difference in meaning is why there is both one and an in modern English. A hammer means ‘I don’t expect you to be able to identify which hammer I have in mind’ or ‘I don’t care which hammer you bring me’.

At the risk of confusing you, let me also say that Joe can say:

I don’t care which hammer you bring me. Bring me one of them.