Malfoy now turned to Hagrid.
"I'm not going in that forest, he said, and Harry was pleased to hear the note of panic in his voice.
"Yeh are if yeh want ter stay at Hogwarts," said Hagrid fiercely. "Yeh've done wrong an' now yeh’ve got ter pay fer it."
"But this is servant stuff, it's not for students to do. I thought we'd be copying lines or something, if my father knew I was doing this, he'd ––“
(Harry Potter and the Sorcerer's Stone)
There is a ‘for’ complementizer in the case. But when considering ‘do’ is a transitive verb, there needs to be ‘what’ or ‘that’. What grammatical explanations are there?