Aber vs Sondern
I was just asked to translate:
"The door is not green, but blue."
I put "Die Tür ist nicht grün, aber blau," but was told that 'sondern' was correct here. Is this right, and if so, when would one use either?
It's difficult to give a general rule. In most cases you translate 'but' to 'aber', but (aber) there are certain constructions in which you have to use 'sondern'. As a rule of thumb: If the main clause is negated and the two alternatives mutually exclude each other, then most probably 'sondern' is the right choice. 'Mein Hund heißt nicht Brutus, sondern Paul', 'Ich mag Bier, aber keinen Wein', 'Ich mag keinen Wein, sondern Bier' (If you're in the process of ordering a drink)'. Note that the alternatives in the latter example don't exclude each other if you're saying that you like beer in general. In this case you could also say 'Ich mag keinen Wein, aber Bier'. EDIT: I made up the beer example in order to point out the importance of mutually excluding alternatives. The use of 'mag' in 'Ich mag keinen Wein' when ordering a drink is a bit colloquial, it's certainly better to say 'Ich möchte keinen Wein'.