Die Synagoge ist schon.
Der temple ist neben der Synagoge.
How does Die Synagoge in the first example become der Synagoge in the second? Is there an easy way to explain this?
Honestly... How did you reach level 25 without asking this question before? There are 4 cases in German : nominative, accusative, dative and genitive.
die, die, der, der respectively for feminine nouns...
Try to put: "German cases grammar" in google, or "German articles tables", but read the theory first to understand what cases are.
neben is a preposition, some prepositions are only Dative and some are only Accusative, but there are also some that are both Dative and Accusative, neben is a dual preposition. so when using neben the noun is either in the accusative or the dative case. thats why Die synagoge turns into Der synogoge in the dative.
If you're using the mobile version of Duolingo to learn, try going on a computer if possible (or at least the Duo website on a mobile browser) and take a look at these pages and read the Tips and Notes: https://www.duolingo.com/skill/de/Dative-Case https://www.duolingo.com/skill/de/Dative-Prepositions
I believe it is because in the sentence: Der Tempel ist neben der Synagoge. Synagoge is here in the dative case. The indirect object in a sentence is called the dative object (die becomes der in the dative)
I am terrible at grammar so I can sympathize with you. When you see something like that in the future ask yourself which case is this noun in; there are 4 cases in German.
Because of your question and my attempting to answering it; I probably will now remember that feminine nouns become der in the dative. And I'll remember what dative means. So, here have a Lingot.
Most of us forget the technical reasons. When I was in high school, I absolutely loved German class - the teacher made sure we had a firm understanding of both German and English grammar.
I remember in an English grammar class, the teacher asked what case a direct object is in. I answered "the accusative case". The teacher answered, "you must take Latin or German. English only has two cases - nominative and objective.
I honestly believe the best way to learn English grammar is by diagramming sentences. In said English class, we did a LOT of diagramming and that helped with learning German.