Go Has No Passive Form

“We will have been gone for five days by the end of our trip.” Is “gone” the main verb or a predicate adjective?

It's a predicate adjective, because “go” has no passive form. We can use “go” in the future perfect tense, but only in the active voice: “We will have gone.” Here, “gone” is a past participle of the verb “go.” When we say, “We will have been gone,” we’re using “gone” as an adjective.

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