We used to see each other _____, but haven’t heard from him since last year.
We’re talking about the piano and the pianist _____ were in the concert we attended yesterday.
The freezing point is the temperature __________ water changes into ice.
Don’t talk about such things _______ you are not sure about.
Hurry! The train _______. You know it _____at 8:30am. .
With the help of high technology, more and more substances _________ in the past years.