You said login is a noun (correct) but your example (login screen) shows it being used as an adjective. The single word can be either an adjective or. When speaking directly to an use i would say you can.
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49 log in is a verb, while login is a noun. There are a lot of questions concerning the correct use if login, log in, etc. I'd pick 1) because the.
Its past tense is logged in (i logged in yesterday).
