關於be... ge問題

This must be your room. – “be” = verb to be. Without the “be” your sentence will have no verb.
That clock must be lying; it isn't noon yet. – “be” = auxiliary verb. “be” in this sentence has the meaning of prediction.
I will be a doctor – “be” = auxiliary verb. “be” in this sentence has the meaning of willingness (to do something / to be someone) in the future.

will / must 皆是助動詞.

沒有了 be, 句子不通.

This must be your room.
That clock must be lying; it isn't noon yet.
I will be ａdoctor 個be 係點, 唔加得唔得? (唔得)
"be" after an auxiliary verb (eg: must) = is. But became positive.
This must be your room (非肯定) 變做 this is your room (肯定句)
That clock must be lying; it isn't noon yet (非肯定) 變做 That clock is lying; it isn't noon yet (肯定句)
I will be ａdoctor = 我將會是一個醫生. (will be = f.t. of am, 不可柝開, 除了 will not be)