If a noun is used to modify or describe another noun, it must always be singular. Careful, there are exceptions to this rule that are tested. And be very careful of trying to find a rule that is always true; this point will depend on the placement of the noun. Let's take a hyphenated noun as our example.
This is a three-year-old child.
The child is three years old.
The 1000-dollar contract was invalid.
The contract was worth only 1000 dollars.
This is a two-bedroom house.
The house has two bedrooms
In the first sentence of each example, the noun phrase (three-year-old, 1000-dollar, and two-bedroom) is modifying the actual noun. But in the second sentence the same noun phrases are objects in the sentence.