As you know, a noun is a person, place or thing, and an adjective is a word that describes a noun:
adjective noun
clever teacher
small office
black horse
Sometimes we use a noun to describe another noun. In that case, the first noun "acts as" an adjective.
as adjective noun
history teacher
ticket office
race horse
The "noun as adjective" always comes first
If you remember this it will help you to understand what is being talked about:
• a race horse is a horse that runs in races
• a horse race is a race for horses
• a boat race is a race for boats
• a love story is a story about love
• a war story is a story about war
• a tennis ball is a ball for playing tennis
• tennis shoes are shoes for playing tennis
• a computer exhibition is an exhibition of computers
• a bicycle shop is a shop that sells bicycles
The "noun as adjective" is singular
Just like a real adjective, the "noun as adjective" is invariable. It is usually in the singular form.
Right Wrong
boat race boat races NOT boats race, boats races

toothbrush toothbrushes NOT teethbrush, teethbrushes
shoe-lace shoe-laces NOT shoes-lace, shoes-laces
cigarette packet cigarette packets NOT cigarettes packet, cigarettes packets
In other words, if there is a plural it is on the real noun only.
A few nouns look plural but we usually treat them as singular (for example news, billiards, athletics). When we use these nouns "as adjectives" they are unchanged:
• a news reporter, three news reporters
• one billiards table, four billiards tables
• an athletics trainer, fifty athletics trainers

Noun as Adjective