As you have noticed, the Green Level is focused entirely on the passive voice. Normally the passive voice is constructed with the verb "be" or "get" and then the past participle. However, there are other similar verb constructions you should know about. This lesson introduces you to the causative form.
have + the past participle
I have my oil changed once every three months.
get + the past participle
I get my oil changed once every three months.
Both sentences have the same meaning. What makes them different from the passive voice is the word order:
S + V + DO + V
Generally, when you use this word order, it sounds like you had some kind of work done for you. You paid someone or got someone to do something and you didn’t do anything.
He got his hair cut by a barber.
(A professional barber did the work.)
The man had the barber
give him a haircut.
Note: If a person doing the work comes after the verb, the verb is in the simple form. If a thing comes after the verb, the verb is in the form of the past participle. In the first sentence, the word "cut" is a past participle. In the second sentence, "give" is in the simple form. This is also explained in the video below.