believed, was believed, were believed, are, was, were

What ______ to be aliens from outer space ______ actually several sheep going astray.

Why does co-reference between pronouns often cause our hesitation in the course of speaking and writing?

Why do most modern English grammarians adopt a two-tense system?

Using inversion:

Come and teach in our school. You won’t find such fine pupils anywhere else.

Using an appropriate cohesive device:

Check out the weather at your destination. If you do not check out the weather at your destination, things can be very embarrassing.

Using extraposition:

Meeting you in London that day was pleasant.

Using a pseudo-deft sentence:

The students are going to write theses.

Using passive voice:

We must put an end to this state of things as soon as possible.

Using an adjunct:

My wife was using the vacuum cleaner. I was trying to listen to a broadcast play.

Using an adjunct:

His mother waited up. He came home.

Using a relative clause:

He said that the men were thieves. This turned out to be true.