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Using a singular noun attributively, as in a five-year plan.
Why you specifically
The rule
English often uses one noun before another noun to show what kind, what purpose, or what related thing it is. In this structure, the first noun is usually singular: a school bus, a customer service plan, a five-year plan, a traffic jam. This is different from Vietnamese, where noun combinations are often formed more analytically and the learner may be tempted to add a plural -s before the second noun. There are exceptions, especially fixed expressions such as sports day or customs officer, but the default pattern in everyday English is singular noun + noun.
The trap
We need a schools bus for the trip.
We need a school bus for the trip.
Bus is modified by the noun school, so school stays singular when it is used before another noun.
She bought a new offices chair.
She bought a new office chair.
When a noun is used as a modifier before another noun, it is usually singular in English.
The company launched a customers service plan.
The company launched a customer service plan.
The noun customer modifies service, so customer stays singular in this compound-like structure.
They made a three-years contract.
They made a three-year contract.
In noun modifiers with numbers, the first noun stays singular: year, not years.
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