Tenses and moods
das Gerundiv
In English: gerundive (zu + present participle)
The pattern zu + Partizip I placed before a noun, with an adjective ending. It has a passive meaning with „must“ or „can“: something that is to be done. It belongs to formal written German. It has nothing to do with the English gerund.
Example: Das ist eine leicht zu lösende Aufgabe. (a task that can easily be solved)