Yes. You can say either "Das ist nicht under Apfelsaft" OR "Das ist unsere Apfelsaft nicht" and it'd make sense. In German, and all laungages, some words are placed after another but when translated, it sounds the other way around. For a tip, when you see sentences like that, don't translate it as: "That is our Apple Juice not".
Very puzzled here. If Apfelsaft is considered masculine singular, then aren't we looking for the accusative masculine singular ending - in this case, -en, or unseren? Ie, 'Das ist nicht unseren Apfelsaft'? If juice is considered plural (I don't believe it is), then it should be 'Das ist nicht unsere Apfelsaft'. It should only be 'unser' if Apfelsaft is neuter, but DL says it's masculine. See http://en.wikipedia.org/wiki/German_declension#definite_articles.
Unser changes based on the case, ie. nominative, accusative, genitive, and dative. I could try to explain this, but since I just had to look it up, I'll provide a link instead. :)
Das ist unser Apfelsaft nicht sounds poetic to me.
In normal German, "That is not our APPLE JUICE" would also be Das ist nicht unser APFELSAFT. -- or you could topicalise unser Apfelsaft by putting it at the beginning of the sentence: Unser Apfelsaft ist das nicht.
"THAT is not our apple juice" would be DAS ist nicht unser Apfelsaft, and "That is not OUR apple juice" would probably best be UNSER Apfelsaft ist das nicht.
When we introduce something new to the conversation in German, we do so with a neuter singular word, regardless of the gender and number of the thing(s) that we are introducing:
- Das ist ein Hund. (masculine) "That is a dog."
- Das ist eine Katze. (feminine) "That is a cat."
- Das ist ein Pferd. (neuter) "That is a horse."
- Das sind Tiere. (plural) "Those are animals."
You'll note that the verb agrees with the new thing(s) in number (ist for a singular new thing, sind for plural new things) but the initial word is always neuter singular, das. (Or it could be dies "this" -- also neuter singular.)
That's (nearly) right.
The gender of a compound word is the same as that of the last part of the compound. So because der Saft is masculine, der Apfelsaft will be masculine as well and thus it is unser Apfelsaft.
And die Flasche (the bottle) is feminine and so for unsere Apfelsaftflasche (our bottle of apple juice) you need unsere because Apfelsaftflasche is feminine like Flasche.
In this case, it's the third part of the word, not the second :) So a better rule is the last part, not the second part.