One(s)

We use ONE(S) to avoid (=evitar) repeating the noun. This works with countable nouns only.

Which is your house?  The one with the white door.

I’d like a hamburger, please. A big one with lots of ketchup.

Can you lend me a ruler? I’m sorry, I haven’t got one.

I’m looking for a flat. I’d like to find a small one with two bedrooms.

I’m looking for a flat. I’d like to find one with two bedrooms.

Can you pass me those CDs? Which ones? The ones on the shelf.

Modern cars are safer than older ones.

One(s) is very often used with adjectives:

I ate the small one (el pequeño).

The yellow one is better (el amarillo).

I bought the large ones (las grandes).

She danced with the ugly one (el feo).

The word one(s) is sometimes omitted immediately after which, this, that, another and superlatives:

Which (one) do you like best?

Let’s have another (one).

That watch is the most expensive (one).