For instance, I'm a native English-speaker who learned German in my teens. Standard German has a pretty firm distinction between Abend and Nacht, the dividing line being the time when most people would go to bed. So if you say something happened letzte Nacht, the assumption of a German-speaking listener would be that it occurred after 11 p.m. or so and probably woke you up, whereas the English phrase "last night" has no such implication. Learning this distinction in order to speak German has affected my English usage. I now say "evening" much more than I used to, even to the point of wishing for a contraction like "yestere'en" to replace the awkwardness of "yesterday evening". (Cf. German gestern Abend.)
So tell me: What are some ways in which a language you learned later in life has had an effect on your native language (whatever you consider this to be)?