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When the predicate is of the 2nd type (see 1.5) and it is of the transitive kind, the clause can be reversed from active to passive form, and backward from passive to active. Example:

On switching from active to passive form, the former object becomes subject: Afghanistan — the object of "the US bombed ..." — comes to the fore as a subject suffering bombing by the US.

And what was the subject in the active form becomes a complement introduced by the preposition by. This complement category is usually known as agent complement, with "agent" meaning something/someone which acts.

Here are three examples of passive clauses where the agent complement (italic) is openly stated:

• The robbers were apprehended by the police.
• Melinda was courted by most of her classmates.
• The XXX Bank was bought out by a large insurance company.

• The robbers were apprehended 3 days ago (this clause features a time complement but no agent complement is stated)
• Melinda is much courted because of her gorgeous blue eyes (a cause complement; again, no agent).
• The XXX Bank has been bought out (no complement of any kind).

When we wish to reverse into active form a passive clause missing the agent, we must add a generic agent. Starting from:

The robbers were apprehended 3 days ago.

The robbers were apprehended 3 days ago by someone.

then we take "someone" as subject in the active form:

Three days ago, someone apprehended the robbers.

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