I am always confused when using a noun and a gerund as an adjective
For example, do we say swim clothing or swimming clothing, electric generator or electricity generator?
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As John Feltz mentions, most of these are idiomatic and each may be constructed differently. You may just have to memorize the words as individual vocabulary, as you come across them. The two you mention should be:
Again, there is no consistent pattern. For example, we say today is a "work day" (a day when most people work), but two people are a "working couple" (both of them work). You can "lend" someone a "helping hand", but you give them "helpful advice". And so on.
By the way, be careful of some resources online that offer lists of these things. In the hope of providing a reference I came across this site, but it is full of errors.