one / ones

Rules

• Rule 1: singular → one

  To avoid repeating a singular countable noun we use one. There is usually the / a / this / that or an adjective before it.

  Pattern: the / a + (adjective) + one

  Example: Which pencil is yours? – The long one.

  Example: My bike is broken. I need a new one.

• Rule 2: plural → ones

  We use ones for a plural noun. The verb is plural too: the ones are …

  Pattern: the + (adjective) + ones

  Example: I like the small dogs more than the big ones.

  Example: The ones on the table are mine.

• Rule 3: Which one? / this one, those ones

  To choose we say Which one? (singular) or Which ones? (plural). this one / that one are singular; these ones / those ones are plural (very common in American speech).

  Example: Which one do you like? – This one!

  Example: Not these cookies — those ones.

• Rule 4: no “a one”, don't mix it up with it

  Without an adjective we just say one: Do you have one? (not ~~a one~~). it points to one particular thing and never follows an adjective: not ~~the red it~~.

  Example: I need an eraser. Do you have one?

  Example: I lost my cap, so I bought a new one.

Common mistakes

• I want the red. → I want the red one.

  An adjective can't stand alone here; add one.

• I need new shoes. These one are old. → I need new shoes. These ones are old.

  Plural → ones.

• Do you have a pen? – Yes, I have a one. → Yes, I have one.

  Without an adjective we don't say a one.