Learn German: Ich bin krank. Darum sage ich das Treffen ab. - I am sick. For that reason I am cancelling the meeting.

Darum is an adverb that connects two ideas, but it takes up Position 1 in the second sentence. In German, the conjugated verb must always be in Position 2 in a regular main clause. So the verb sage comes right after darum, pushing the subject ich to Position 3.

Could I use deshalb or deswegen instead of darum here?

Yes! Darum, deshalb, and deswegen all mean "therefore" or "for that reason." They are interchangeable here and follow the exact same word order rule, so you could absolutely say Deshalb sage ich das Treffen ab.

Why is the word ab sitting at the very end of the sentence?

It belongs to the verb absagen, which means "to cancel." Absagen is a separable verb. When you use it in a normal present tense sentence like this one, the core verb sage goes in Position 2, and the prefix ab gets kicked to the very end of the clause.

Can I combine these two sentences into one using a comma?