Learn Russian: Капуста на тарелке.

In Russian, the present tense of the verb "to be" (am, is, are) is almost always left out. You just say Капуста на тарелке (Cabbage on plate), and the "is" is automatically understood.

How do we know it means "The cabbage" and not "A cabbage"?

Russian doesn't have words for "a" or "the" (articles). You have to figure out which one makes sense from the context. Here, we translate it as "the" because we are likely talking about a specific cabbage resting on a specific plate.

Why does the word for plate end in -е (тарелке) instead of its normal dictionary ending?

The dictionary form of the word is тарелка. Because the plate is the location where the cabbage is resting, we have to change the word to show location (called the Prepositional case). For most feminine nouns ending in -а, you do this by changing the -а to an -е.

Why do we use на here instead of в (in)?

На means "on" or "on top of", while в means "in" or "inside". Since a plate is a flat surface and the food sits on top of it rather than enclosed inside it, Russian uses на.