It is pretty easy to find the main verb.
All you have to do is to find what is the sentence trying to convey you mainly and plays the biggest role.
I used to hurt people.
The main verb is hurt because its main agenda is to tell you that you hurt people.
**e.g.** _I love to play video games whenever I am free._
"love" is the main verb since my agenda to tell you is what I love — play video games.
Another way to identify is to test if it can standalone (not recommended),
i.e. I love video games.
**e.g.** _Many high school students, some of whom are aged 12 only, have a part-time job._
The main verb is "have" since my agenda is to tell you many high school students have part-time jobs, others are extra.
many high school students → have a part-time job
So, it is fine to be standalone too.
i.e. Many high school students have a part-time job.
**e.g.** _This sentence will tell you that it is written in English Language._
Yup, "will" plays the biggest role. However, it is just a modal verb. So, it is disqualified.
What about "tell"? No, it is just extra since it does not refer to the subject, "this sentence".
The answer is "written".
Standalone: This sentence is written in English Language.