Learn French: C'est une forme carrée.

When you say 'it is' followed by a noun with an article (like une forme), you must use c'est. If you were using an adjective all by itself to describe a shape you already mentioned (like saying 'It is square'), you would use elle est (elle est carrée). But because the article une is included here, we have to use c'est.

Why does the adjective carrée come after the noun forme?

In French, most adjectives are placed after the noun they describe. Only a specific group of common, short adjectives goes before the noun (usually related to Beauty, Age, Number, Goodness, or Size—often taught as 'BANGS'). Words that describe shapes, like carrée (square) or ronde (round), follow the normal rule and go after the noun.

Why does carrée have an extra 'e' at the very end?

It needs to agree with the feminine noun forme. The default masculine spelling is carré (which already ends in an accented é). To make it feminine, we add a second e to the end. This extra e does not change how the word is pronounced.