My question is using nouns as modifiers.the name that acts like an adjective should be singular most of the ti

consider these examples:1-flowers that are grown in gardens are called: garden flowers 2-workers who go to work at nigh are called night workers 3- shops that sell books are called book shops 4-papers that have news are called :newspapers 5-stories that are about wars are called war stories 5-factories that make shoes are called shoe factories .....as you see we dont know when we should make the modifier ( the first noun singular -in fact sometimes we make it singular like shoes and sometimes we do not change it like news ,physics ) how should i me myself as a non-native english teacher that never faces native people should know this?if i know a point i know how masterfully deliver it to my students. does this point (nouns as modifiers )has a special rule or you decide it according to what you have heard among people? what should we do with this sentence? shops that sell jeans-shops that sell glasses -shops that sell gloves,scissors,.... and i warmly thank those dear natives

My question is using nouns as modifiers.the name that acts like an adjective should be singular most of the ti
A general rule is that the modifier should always be singular. You encounter exceptions to this rule when the modifier is a noun that is rarely or never used in the singular form -- news, scissors, eyeglasses, jeans. There are more words that behave like this (glove and shoe do not count because they are commonly used in their singular form) and you pretty much just have to memorize them.