Is subjunctive usage justified here?

grammaticality subjunctive-mood

"Watch our father DO a back flip" (Subjunctive or Infinitive?)

Do is an infinitive but the sentence is not idiomatic. (Note the present participle.)

"[I am] Watching father do a back flip."

As basic guidance, the verb in answers takes the same form as the verb in the question, and the question was "big brother asks him what he is doing?

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