month as an adjective [duplicate]

Those are called compound and hyphenated compound adjectives. And adjectives don't have plural forms.

Additional examples

three-storey building (three-stories bulding)

four-wheel drive (four-wheels drive)

32-bit processor (32-bits processor)

Some adjectives can only be used to modify nouns, for example the adjective indoor. We can talk about:

• indoor swimming pools

But we don't usually say:

• *The pool was indoor (not good)

We call adjectives that appear before nouns attributive adjectives. The adjective indoor is called an attributive only adjective.

Other adjectives can't usually be used before a noun. We usually find these adjectives as the complements of verbs like BE, FEEL or BECOME. So we can say

• She was afraid
• She felt afraid

But we cannot say:

• *an afraid girl (not good)

Adjectives that we use like this are called predicative adjectives. The adjective afraid is a predicative only adjective.

We can use most adjectives as attributive adjective and predicative adjectives:

• a huge elephant
• The elephant was huge.

Sometimes we have two adjectives that look similar and mean the same thing. One of them is attributive only, and the other predicative only. For example, the adjectives live and alive. When these words are used to describe things that aren't dead, we use live as an attributive adjective and alive as a predicative adjective:

• a live snake
• The snake was alive.
• *an alive snake (wrong)
• *The snake was live. (wrong)

The term two-year old is used as an attributive only adjective phrase:

• a two year old whisky
• *The whisky was two year old (wrong).

The term two years old is used as a predicative only adjective phrase:

• *a two years old whisky (wrong)
• The whisky was two years old.

Similarly the following measure phrases only have attributive uses, where the noun part of the phrase has no plural inflection:

• twenty pound
• ten mile
• five minute

as in the following examples:

• a twenty pound note
• a ten mile journey
• a five minute meeting

In contrast, the following measure phrases where the noun part is plurally inflected can only be used predicatively:

• the meal was twenty pounds
• the journey was ten miles
• the meeting was five minutes

Note

We can also use the term two year old as a nominal phrase. We can use it like a noun.:

• I have two children: a two year old and a three year old.

Notice that we use the attributive adjective here because we mean: a two year old child.

Hope this is helpful!

On the other hand, you could describe a successful tennis player equally as "a five-times winner of the Australian Open" and "a five-time winner of the Australian Open". And the former British government minister John Prescott, who was nicknamed 'Two Jags' (because he had a Jaguar as an official vehicle in addition to the Jaguar he owned), was sometimes referred to as 'Two-Jags Prescott', not 'Two-Jag Prescott'. So the general rule is not absolute.

I think it all goes back to the -a suffix in Old English that marked plural adjectives, but which has since been lost. Perhaps if we'd had one a thousand years ago, we'd have called it A ten pounda note.

Switching from money to weight, some people would ask the grocer for Ten pound of apples. Presumably that's by association with the form we're talking about here, but I imagine most of us would use the plural in this case. So it's certainly not all cut-and-dried.

In short, it seems this is another case of 'language on the move' in ways we don't normally notice.