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 gles -shops that sell gloves,scissors,.... and i warmly thank those dear natives