Which one is grammactically correct?

Test Preparation SAT Preparation
1

<p>I started to work on…</p>

<p>or</p>

<p>I started working on…</p>

<p>My friend asked me that today and I wasn’t 100% sure. I just wanted to double check because grammar isn’t my best subject.</p>

2

<p>both are fine</p>

3

<p>Yeah, no problems with either to my knowledge.</p>

4

<p>I prefered “start working” ^^
But yes, both “start working” and “start to work” are fine</p>