Learn Dutch: Het voortdurende schreeuwen is verschrikkelijk.

This is a substantival infinitive! In Dutch, you can take the entire infinitive of a verb (like schreeuwen) and use it as a noun to describe the activity itself. In English, we usually do this with '-ing' words (shouting, walking, waiting). Every infinitive used this way becomes a neuter noun, which means it always takes the article het.

Why does the adjective voortdurende get an -e at the end?

Because schreeuwen acts as a het noun here, it follows normal adjective rules. When a het noun is preceded by the definite article het, any adjective right in front of it gets an -e ending. The base adjective is voortdurend (constant), so it becomes voortdurende.

If voortdurend is the base adjective, isn't that actually a verb form too?

Exactly! It comes from the verb voortduren (to last or continue). By adding a -d to the infinitive, you get the present participle (voortdurend), which means 'lasting' or 'constant.' Dutch frequently uses these as normal adjectives.

Why doesn't verschrikkelijk (terrible) get an -e ending too?