Question

add a clause to expand the sentance meaningfully we saw an aeroplane .........​

Answer 1

Answer:

This sentence, we saw an aeroplane, consists of a single independent clause. In order to increase the length of the sentence one needs to add a subordinate clause preceded by a conjunction.

So the sentence will be-

We saw an aeroplane until it flew high and disappeared  into the clouds.

In the new sentence we have added a subordinate clause to make the sentence more meaningful.

Hope it helps :)