The sentence "We saw him ... over a wall as the police approached" requires the correct verb form after "saw him".

• When verbs follow "see, hear, watch, notice" in an active sentence structure (subject + verb + object), we use either:
  1. The base form (bare infinitive) without "to" → We saw him leap over a wall.
  2. The present participle (-ing form) to describe an ongoing action → We saw him leaping over a wall.

The present participle is used in this case because, it helps to show that the action of "leaping" was an action that was ongoing as the police approached.