Debere or Gerundive?

Latin Learning Latin
1

When ought one use debere when indicating what someone should do versus using a gerundive?

For example: I should do this.
Debeo hoc facere.
OR
Hoc mihi faciendum est.

Aside from the fact that one construction is active and one passive, are there any other considerations?

2

I believe the rule is that one uses debere when expressing moral obligation. The gerundive is used in nearly all other instances, I think.

Debeo eum iuvare. “I ought to help him.”
Haec epistula est scribenda mihi. “I have to write this letter.”

3

Thanks Deccius!