My parents _______ have arrived at suppertime but their flight was held up due to the heavy fog.
—Did you look up the time of trains to Shanghai? —Yes, the earliest train is________ to leave at 5:30 am.
She stepped into the bedroom quietly_______ she might wake up her roommates.
Many people have been ill from a strange disease these days,________ we’ve never heard of before.
—He_______ to the meeting. Have you informed him of it? —Sorry. I________to.
It is not enough only________ the rules of grammar if you want to learn English well.