What is correct form of writing: "users names" or "user names"? [duplicate]

Solution 1:

What words should I use to name such collection of names of users: user names, or users names?

In the second example, users names sounds like user's names. You are already pluralizing names; I believe you are modifying names with the adjective user.

Using another adjective-noun pair might help clarify the situation:

sentence structures vs. sentenses structures

The latter is very odd. Structures is being modified by sentence. That is enough information.

Edited by request: There is too much information imparted in the second example. The thing, the noun, we want to emphasize is the head (or final) word: structures. The word sentence here is an adjective. It doesn't need to be plural.

Again, if we were writing about a more than one horse farm, we would say horse farms to indicate that the most important word (which is farm) is plural. It would just confuse things to say horses farms. It would only make sense if the farms belonged to the all of the horses. Obviously they do not, or there would be an apostrophe: horses' farms.

Your 'names' do not belong to all of the users collectively. Each has his own user name. Together , they are user names.

Solution 2:

Please allow me to answer this question by quoting from a grammar book: Practical English Usage by Michael Swan. According to 531 singular and plural (9): noun + noun:

531.1 first noun singular: shoe shop
In noun + noun structures, the first noun is normally singular in form even if it has a plural meaning.
  a shoe shop (= a shop that sells shoes)
  a toothbrush (= a brush for teeth)
  trouser pockets (= pockets in trousers)
  a ticket office (= an office that sells tickets)

531.2 exceptions
Some nouns are plural in this structure. These include nouns which have no singular form (like clothes), nouns which are not used in the singular with the same meaning (like customs), and some nouns which are more often used in the plural than in the singular (like savings). In some cases, e.g. antique(s), drug(s), usage is divided, and both singular and plural forms are found. In general, plurals are becoming more common in this structure. Examples: [...]

Note also that singular nouns ending in -ics can be used before other nouns.
  athletics training
  an economics degree

We use the plurals men and women to modify plural nouns when they have a 'subject' meaning; man and woman are used to express an 'object' meaning. Compare:
- men drivers (= men who drive)
  women pilots (= women who fly planes)
- man-eaters (= lions or tigers that eat people)
  woman-haters (= people who hate women)

The last four examples are especially helpful: user in username is used to express an 'object' meaning. A username means a name of a user, not a user who names something.