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.
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**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._
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**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._
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**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._