How tax on basic and commssion works?

Hi,

I'm highly Confused by tax. So I'm hoping someone can help explain it better.

So I earn a basic salary of 19000 annual however I also earn commission which can vary each month and an annual medical health contribution. Because cinnamon can vary so much I like to be able to calculate what I earn.

I've tried calculating my take home pay based on 20% tax. But it's never that high. I've noticed I'm charged between 11 - 13% tax depending on what I earn. Can anyone explain why I'm only on that instead of 20% (not that I'm complaining)

The reason I ask is because I've recently been promoted to 20947 Euro annual salary and commission will be added monthly.

I'm not very good with numbers and understanding taxes, it looks like a different language when I look at tax certs, so please if possible try dumb it down for me ha.