What does "Forget expelled" mean?

Question

"Did you really blow up your Aunt, Harry?" said Hermione in a very serious voice.

" I didn't meant to," said Harry, while Ron roared with laughter. "I just-lost control."

"It's not funny, Ron," said Hermione, sharply. "Honestly, I'm amazed Harry wasn't expelled."

"So am I," admitted Harry. "Forget expelled, I thought I was going to be arrested." He looked at Ron. "Your dad doesn't know why Fudge let me off, does he?"

Harry Potter and the Prisoner of Azkaban

I don't understand how "forget expelled" is used here.

If the writer dropped the subject "I"(Harry), then the verb has to be "(I) Forgot", not "forget". But it still doesn't seem right. Doesn't it have to be "Forgot (about) expulsion" or "Forgot about being expelled" or something like that..

Or is it an imperative? Then how come "forget" is followed by "expelled"? I don't know what it means here.

Answer 1

In this case, forget is imperative. Dictionary.com defines forget as

5. to fail to think of; take no note of

When Harry says forget expelled, he means that, in that instance, being expelled was the best thing that could happen or the least of his worries (because he thinks that being arrested is worse than being expelled).

Answer 2

Kman3 has explained that forget is imperative. Your question also asks why expelled does not need to be expulsion or being expelled, since it is the direct object of the imperative verb forget. To understand this, I think it is best to imagine that Harry is effectively quoting Ron. In other words, imagine that he is saying:

Forget "expelled", I thought I was going to be arrested.

This explains why Harry does not change the word. (You would not usually write the quotation marks here, it is just a helpful way to understand the structure.) Note that this pattern is much more common in speech, and can require a lot of intonation to make the meaning clear. Here is an example that is even more exaggerated, a dialogue between two criminals after they committed a bank robbery.

Criminal #1: I'm worried that the clerk saw my face.

Criminal #2: Forget saw your face -- I dropped my driver's license at the scene of the crime!

Of course, forget saw is completely ungrammatical, but thinking of this as Forget "saw your face" explains this unusual combination of words. (You might ask why he didn't say "forget saw my face"; I don't have a good answer, but that sounds less natural.)