Both means a single thing, but we use "one" when we're implying that there could be more of the same thing.

The main difference between "a" and "one" is:

• "A/an" means a single thing. We typically use it to introduce something that hasn't been mentioned before.

  A benefit of eating well is that you can maintain a healthy weight.

• "One" also means a single thing, but we use it when we're implying that there could be more of the same thing.

  One benefit of eating well is that you can maintain a healthy weight (It gives the impression that there is more than one benefit.)

The second difference is:

• "A/an" is always followed by a singular countable noun. It's a determiner.

  She ate a banana with chocolate.

• "One" may or may not be followed by a noun. It can be a determiner or a pronoun.

  How many books did you read last year? I read only one book. (Determiner)

  She's having a coffee. Can I have one? (Pronoun)

If you just want to introduce a noun for the first time, use a/an, not one.

Can I have an ice cream?

If you remove the noun, use the pronoun "one" instead.

Can I have one?

Can I have a?

Use "one" in a situation like this:

How many books did you buy? I only bought one book.

Note the difference:

Focus on one thing at a time to get more done and feel less stressed. (Focus on not more than one thing)

If a person focuses on a thing for a long time, he can become an expert at it. (We're introducing the noun ("a thing"), not quantifying it.)

Another example:

After buying three sets of underwear and one T-shirt, I had a cup of coffee. (One T-shirt, not two)

When using an adverb like "only" or "just", we usually say "one" (e.g., "Mary has only one regret in life.")

How do you know whether to use "a" or "an"? Use "a" if the following word begins with a consonant sound. Use "an" if it begins with a vowel sound. Keep in mind that this rule is based on pronunciation, not spelling. For example, the first letter of the word "Euro" is a vowel, but it has a consonant sound (a "you" sound). As a result, it takes the indefinite article "a" (not "an").

Can I open a euro bank account in the US?