Pluperfect, why de in the following sentence

Hi John,

The verb format is "oublier de faire (qch)"  --> to forget to do something

So in "J'avais oublié de finir ...."     you will see that the format above is being followed and the "de" is part of the verb format in this case.  "I had forgotten to finish my homework" 

Hope this helps.

Bonne continuation.

Jim