Learn Arabic (Modern Standard): العرض ممتاز ما دام الفريق منسجما. - The show is excellent as long as the team is harmonious.

Because it is the predicate (khabar) of ما دام. Just like كان (was), ما دام changes the predicate of a nominal sentence into the accusative case. The subject, الفريق, remains in the nominative.

Does the ما in ما دام make it a negative verb?

No, the ما here isn't the negative particle "not." Instead, it is an adverbial particle that combines with the verb دام (to last or continue) to create the meaning "for the duration that" or "as long as."

What form is the word منسجما, and how does it relate to its root?

It is the active participle (ism faa'il) of the Form VII verb انسجم (to be in harmony, to flow together), coming from the root س-ج-م. Form VII often describes an action happening naturally or states of being, making it perfect for describing a team in a state of harmony.

If we moved ما دام الفريق منسجما to the beginning of the sentence, would it still work?

You can start a sentence with ما دام, but to say "As long as the team is harmonious, the show is excellent," you would usually need to add a result marker like فـ (fa-) to the main clause, making it ما دام الفريق منسجما فالعرض ممتاز. Keeping the main clause first, as in this sentence, is often simpler and more common.