Is “Login” One Word or Two?

Yes. But also, no. Wait, where are you going? I'm not being difficult on purpose; that's just my default setting. What I'm saying is that “login” and “log in” are both correct. It just depends on how you're using them in a sentence. 

If you’ve been reading any of my other guides, you already know that the English language is the very definition of logic, reason, and consistency. *checks previous guides* Scratch that. The English language is a fickle bastard that does what it wants when it wants with no regard for your mental or emotional well-being, which are clearly hanging on by thin fucking threads. 

“Login” is just one of the many words that change form depending on if it’s being used as a noun or as a verb. It can be a thing in one sentence and an action in the next, so it’s easy to confuse the two—or just say “fuck it” and decide to only use one form for every instance for the rest of eternity. If that’s your choice, I wish you well on your life’s doomed grammatical journey.

When to use “login” 

You’ll want to use the single-word version (“login”) when it’s a noun—meaning it’s probably the subject, direct object, or indirect object of your sentence.

Example:

Your login credentials have been compromised because you use the same password for every website on the internet.

When to use “log in”

You’ll want to use the two-word version (“login”) when it’s a verb—meaning it’s an action being performed.

Example:

I tried to log in to my account but it said the username “s3xym@n69” wasn’t recognized.

Note that the above example uses “log in to” and not “log into.” In this context, “log in” is the verb, not “to log.” You’re not a lumberjack harvesting trees to turn their flesh into butt napkins!

These same “rules” apply to plenty of other words: setup vs. set up, lineup vs. line up, knockout vs. knock out, etc.

And now, back to your regularly scheduled disregard for the finer points of grammar. Have fun, you little grammanarchists!