Learn Portuguese (Portugal): A forma do bolo é redonda.

The adjective redonda describes the subject of the sentence, which is A forma (the shape). Because forma is a feminine noun, the adjective must also be feminine, even though it sits right next to the masculine word bolo.

Why do we use é instead of está for "is" here?

We use é (from the verb ser) because a shape is considered a defining or permanent characteristic of this cake. We use ser for permanent traits, while estar is used for temporary states like emotions or locations.

How does the word do work in this sentence?

do is a contraction of the preposition de (of) and the masculine definite article o (the). Portuguese does not use an apostrophe-s for possession, so "the cake's shape" translates literally as "the shape of the cake" (a forma do bolo).

Could the word forma mean anything else in a baking context?

Yes! In Portuguese, uma forma is also the word for a baking pan or cake tin. Depending on the conversation, A forma do bolo é redonda could just as easily mean "The cake pan is round."