Whenever we compare two things or persons or situations, we use comparative forms like heavier, thinner etc. But with some words it is often confusing whether to add ‘er’ in the end or add ‘more’ in the beginning.

Should I work more hard? Or
Should I work harder?

He is cleverer than his sister. Or
He is more clever than his sister.

She is gentler than her brother. Or
She is more gentle than her brother.

Rohan is quieter than Mahi. Or
Rohan is more quiet than Mahi.

We should try and go a little farther. Or
We should try and go a little further.

You need to be more friendly with people. Or
You need to be friendlier with people.

Well, if you are still confused here are the answers:
It is ‘harder‘ which is the correct form. So, Should I work harder? is correct.
For other words, both the forms are acceptable. So it could be
Cleverer as well as More clever
Gentler as well as More gentle
Quieter as well as More quiet
Farther as well as Further
Friendlier as well as More friendly