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For example lets consider the name 'file system'. It consists of 2 nouns. Now, lets assume I want to use it in plural form. Which one of the following sentences is correct?
I have 5 file systems installed.
I have 5 files systems installed.
Is it like always one of them is valid, or maybe it depends on the context?
Context is not relevant. The correct version is always this:
I have 5 file systems installed.
The rule is that only one word of a compound noun gets a plural ending. In most cases, it is the last word. In a few rare cases, where the final word modifies the previous noun, the noun gets modified:
courts martial
not
*court martials
where court is the noun, and martial identifies the kind of court. As I said, these cases are rare. As far as I know, everyone memorizes them individually.
