There is a strong tendency in American English to smoosh the halves of hyphenated word and phrases together and drop the hyphen, so we commonly see phrases such as “enter your login and password.” This is a misuse of “login” since logging in involves entering both your ID and password, and “login” is not a proper synonym for “ID” alone, or “user name”—commonly abbreviated to the ugly “username”. Such mash-ups are influenced by the world of computer programming, where hyphens and spaces are avoided.
If you would prefer to use more standard English, it would be appropriate to use “log-in” as the adjectival phrase: “Follow the correct log-in procedure.” But the verb-plus-adverb combination should not be hyphenated: “Before viewing the picture of Britney you’ll need to log in.”
“Log on” and “log-on” mean the same thing as “log in” and ”log-in” but are less common now.
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