Indirect objects that answer the question "by whom"

Your English teacher should cut wood but not teach English.

There are two special verb constructions with accusative (= direct object in English) + a verb form. (Actually there are more but I don't want to write a grammar here.)

1 - I saw the bridge explode. (noun + bare infinitive, emphasis on fact).

2 - I saw the bridge exploding (noun + present participle / gerund, it can be either form). In a film this would correspond to a close-up, evoking the scene of the exploding bridge. You can derive such a construction, eg by saying:I saw the bridge at the moment of exploding - or: I saw the bridge, it was exploding.

The verb construction verb + noun + bare infinitive is 2000 years old. It was one of the favourite constructions of Roman authors. And an English teacher should know this construction and not tell fairy tales.

When I got to the three log doorsteps I heard them unlocking and unbarring and unbolting.

Your English teacher is wrong, I’m afraid.

There is no indirect object in your example. "Them" is the direct object of "heard". The participial clause "unlocking and unbarring and unbolting" is not the direct object, not an object at all, though it is a complement of "heard". Such clauses are often called 'catenative', from the Latin word ‘catēna’ meaning chain.