Learn German: Der Kuchen mit süßen Erdbeeren ist sehr lecker. - The cake with sweet strawberries is very tasty.

The preposition mit always requires the dative case. We are talking about plural strawberries (Erdbeeren) without an article like 'the' or 'some' in front of them (the zero article). For plural nouns in the dative case with no article, the describing adjective takes the -en ending, making it süßen.

Does the noun Erdbeeren have an extra -n added to it because it is in the dative plural?

Actually, no. While it is true that plural nouns in the dative case usually add an extra -n at the end, the standard plural of die Erdbeere (strawberry) is already die Erdbeeren. Since it already ends in an -n, you do not add another one.

Why doesn't lecker have an ending like süßen does?

Adjectives only get endings when they are placed directly in front of a noun (like süßen Erdbeeren). Here, lecker comes after the verb ist to describe the cake. Adjectives used this way always stay in their basic dictionary form without any endings.

Why is mit süßen Erdbeeren placed right after Der Kuchen instead of at the end of the sentence?

The phrase mit süßen Erdbeeren describes the cake itself, so it needs to stay right next to Der Kuchen. If you put it at the end of the sentence, it changes the meaning slightly to suggest a plain cake is delicious when you eat it alongside some strawberries, rather than it being a cake baked with strawberries.