I really don't know if they ____ ____a party tomorrow.If they____I really don't know if they will have a party tomorrow.If they____(do还是have),I will call you.

I really don't know if they ____ ____a party tomorrow.If they____I really don't know if they will have a party tomorrow.If they____(do还是have),I will call you.
I really don't know if they ____ ____a party tomorrow.If they____
I really don't know if they will have a party tomorrow.If they____(do还是have),I will call you.

I really don't know if they ____ ____a party tomorrow.If they____I really don't know if they will have a party tomorrow.If they____(do还是have),I will call you.
do
【语法上讲,have 也没错,但是最优,选do,如果还能选,最优 是选will.
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