The part of the sentence "he completely forgot" is correct and usable in written English.
You can use it when referring to someone forgetting to do something or forgeting something that has happened. For example, "John knew he had to leave early for his flight, but he completely forgot".
Over the summer he completely forgot about the album and went sailing for six weeks with family and friends.
He completely forgot that the ideology he so much wanted to be part of regarded him as one of its most hated enemies.
"With Tiger, it was almost like when the ball went in on the 18th hole on Sunday, he completely forgot about it.
But you won't know that until you're in it.'" Mr Bispham confirms that he completely forgot about the cameras, with one exception: "I caught the camera in the corner of my eye and had this weird feeling: if I fall over now that could be really bad.
In fact, he completely forgot the words "birth control".
The actor is currently in London working on another project and says he completely forgot it was Emmy nominations day.
He goes to stay with his cousin Mahmut (Muzaffer Özdemir), and you can tell how eagerly Mahmut is awaiting his kinsman from the fact that he completely forgets to stay home and let the poor guy in.
Hughes plays a decent forward defensive against Anderson and is so unnerved by this unusual display of competence that he completely forgets about the ball that might be bouncing back towards stumps.
When God forgives you, he completely forgets that you ever sinned.
When he died 40 years later, he was completely forgotten, and the works we have come to think of as his greatest ("Billy Budd," "The Confidence Man," "The Piazza Tales" and "Moby-Dick" itself) languished in oblivion.
Apparently, in his excitement, he had completely forgotten where he had put them.
