Some esoteric grammar talk coming up. Run and hide.
I haven’t seen this addressed anywhere else, but maybe I haven’t looked hard enough.
Do you “sign into” the web site, or “sign in to” the web site? It’s used both ways all over the place, but in my opinion, only one way makes sense.
“Sign in” is a phrasal verb
“Sign in” (like “look up,” in the sense of “consult a dictionary”) is a phrasal verb. In other words, both “Sign” and “In” are necessary to complete the verb.
In other words again, “I sign in to see my account” is different from “I sign in ink.” The action in the former is “Sign In” and in the latter, “Sign.”
Getting into the web site is a matter of signing in. Not signing. So you keep both “sign” and “in” just as they are, without changing the “in” to an “into.” Am I making sense?
How about this conversation:
A: Where are you going?
B: I’m going into this bar.
A. Where are you signing?
B. I’m signing into this web site.
A. Where are you signing in?
B. I’m signing in to this web site.
The first and the last make sense. The middle one doesn’t.
By the way, this is why both words should be capitalized in a headline, even if prepositions are otherwise not:
“Robert Signs In to His Account” vs. “Robert Learns Semaphore and Signs into the Sky”.