To use the words in my title, I would log in (verb) to my account, but I would need a login (noun) in order to do this. I would go work out (verb) at the gym and hope to have a great workout. If I were an undercover cop, I might have to stake out (verb) a place and want to make sure I was awake for my stakeout (noun). In these instances, the concept of each of these words, regardless of whether they're nouns or verbs, is the same--in the case of login versus log in, they both have to do with the act of gaining access to something, usually an account of some sort that can be accessed electronically. The compound word as a whole can also function as an adjective rather than a noun, as in, "I need a login ID for that." It's not just any ID, but specifically a login ID that's required, so in this case, an adjective was required to specify the type of ID. Other words I can think of off the top of my head are setup/set up, pickup/pick up, backup/back up.
There are some cases in which the noun and verb might not be referring to the same concept. A confusing one might be makeup versus make up. I might go and make up my room, and that would mean I would tidy it and make the bed. If I look at the room makeup, I would be looking at how the room is designed or laid out. This is not to be confused with the makeup I put on my face, and yet I can go make up my face, and that would be the verb to signify I will apply cosmetics to my face. How do you like them apples (to use a colloquial phrase)?
If you ever get confused about it, the verbs do function as phrasal verbs. That's why I log in to my account, rather than log into it, because "to log in" is the verb, and in this instance, "in" doesn't function as a preposition, so it can't be combined with "to" in order to create the preposition "into". My rule of thumb, if you're not sure if the word functions as a noun or a verb when you want to use it, try to use the past tense. For example, you couldn't say: I loginnned to my account, or I workouted at the gym. You also couldn't say you loggedin to your account or that you workedout at the gym. These are clearly not correct, so that means the word should be separated into its component parts so that you are using the verb and be able to log in to your account and work out at the gym.
I see these words written out incorrectly all the time, and it drives me batty! This is why I have to rant about it here, I think. It's such a therapeutic outlet ;o)