How do you say "under an angel's wing" in Latin? Should it be sub ala angelus or sub ala angeli? A friend of mine has a tattoo (using the 1st option), and she's afraid it's not correct.
The first phrasing you quote has the nominative case angelus of the Latin word for an angel. Thus it means roughly:
Sub ala angelus.
[There is] an angel under [a/the] wing.
Latin often leaves out "is" and doesn't need "there" or an article. The phrase makes sense, but the wing belongs to someone other than the angel.
The second one uses the genitive case angeli. This gets the meaning correct:
Sub ala angeli.
Under the wing of an angel.