The indefinite article a/an is used:

• with singular countable nouns when we talk about them in general. / need a new handbag.

• with the verbs to be and have (got). Jane has (got) a cat. It's a Siamese.

• before Mr/Mrs/Miss/Ms when we refer to an unknown person. A Mr Wells is waiting for you.

• to show price in relation to weight.

The tomatoes cost one pound twenty a kilo.

• to show distance in relation to time. My car can go 250km an hour.

• to show frequency.

/ work at the library three days a week.

We do not use a/an:

• with uncountable nouns or plural countable nouns. We use some instead of a/an.

We bought some chocolate and some walnuts.

This dress is well-made. But: This is a well-made dress.