What part(s) of speech are "or else" and "otherwise" and why is "otherwise" more flexible if it's the same part of speech?

Solution 1:

Traditionally, 1b requires a semicolon or full stop:

Clean your room. Otherwise I will ground you.

The reason is that otherwise is traditionally considered an adverb, and two sentences can traditionally only be joined with a mere comma if there is a conjunction between them:

$ I don't like him, consequently I will sack him.

This is traditionally considered a "comma splice", which is undesirable in formal English. Consequently is an adverb and not a conjunction.

I don't like him, and I will therefore sack him.

Here the conjunction and makes the comma possible.

Clean your room, or else I will ground you.

Or is a conjunction, so no problem there.

? Clean your room; I will ground you or else.

Or else is two words, and hence two parts of speech, not one. The problem here is that else is traditionally considered an adverb, and so it has to be part of the sentence that it modifies; in other words, it tells the reader that the threat I will ground you is the "other" ("else") possibility, and so it should be part of that clause. If you put else after or, it becomes part of the clause that starts after or; after all, or is a conjunction here, and it begins a new clause.

Informally, you can end a sentence with or else, but that is elliptical, and the "other" possibility (the threat) is omitted:

Clean your room, or else...

In your example, the threat is explicitly mentioned, so else needs to be in the same clause as the content of the threat.