The old man [is used to walk] to the market every morning.

'Be used to' means 'be accustomed to' and is followed by a gerund, so 'is used to walking' is correct. 'Used to walk' (without 'is') would show a past habit, which is the trap. 'Uses to walk' and 'was using to walk' are not idiomatic. Read each replacement into the sentence and choose the one that is grammatical, idiomatic and keeps the original meaning; choose 'No improvement' only when nothing is wrong.