Learn German: Der zu verfassende Text muss perfekt sein. - The text that is to be composed must be perfect.

This is the Gerundivum (also known as the zu-Partizip). It is formed by putting zu in front of the present participle of a verb (verfassend). When placed before a noun like an adjective, it takes on a passive meaning combined with a sense of necessity or obligation. So, der zu verfassende Text literally translates to "the to-be-composed text."

Why does verfassende have an -e at the end instead of just being zu verfassend?

Because it functions exactly like a normal adjective describing the noun Text. Since Text is a masculine noun in the nominative case (it's the subject of the sentence), and it follows the definite article der, the participle takes the standard weak adjective ending -e.

How would you express this same idea using a relative clause instead of the Gerundivum?

The Gerundivum is a very elegant, compact structure mostly used in formal written German. If you wanted to unpack it into a standard sentence, you would use a passive relative clause with a modal verb: Der Text, der verfasst werden muss, muss perfekt sein.

Could I just use schreiben instead of verfassen here?