I always thought login is the noun and log in is the verb. For example, "you need to log in using the modal login window". Is this really the case or can they be used interchangeably? Does logout and log out follow the same rule?
"Login" is a noun or adjective. "Log in" is a verb.
Go to the login page. - adjective
Go to your login. - noun
Log in to twitter. - verb