[StringEpsilon]

How a word is affected by grammatical rules (such as pluralization) is not determined by etymology (but it can be).

Here's the german conjugation of "mailen" (writing an e-mail), borrowed from "to mail":

Ich maile, du mailst, er/sie/es mailt, wir mailen, sie mailen.

I don't know any loanwords that break english pluralization rules in german, but for the reverse: The correct plural for "Kindergarten" would be "Kindergärten" (not "kindergartens"), which I imagine some english speakers would have problems with. And "Autobahnen" is rather unintuitive compared to "autobahns".

[germanier]

> I don't know any loanwords that break english pluralization rules in german

The plural of 'Baby' is 'Babys' (instead of 'Babies' – though some people also use that form) and 'Computer' doesn't change.

On the other hand, both 'Indices' and 'Indexe' are used and for 'Tempus' the only plural is 'Tempora'.