Question

no sooner did the thief see the policeman then he took to his heels. ( make it affirmative)

Answer 1

As soon as the the thief saw me than he took to his heels

Answer 2

An affirmative sentence must always be positive, without any negative words (such as no, not, doesn't, wasn't, isn't and never). Sentences like this always state a fact or a positive opinion about a person, thing or place.

The affirmative form of the sentence in the question is:

As soon as the thief saw the policeman, he took to his heels.