i have a 2009 2.0 TDI 140 hp Golf. I am going to buy some parts but it's hard to tell what MK it is. Where can I find info about that? It think it is a MK5, but some people say MK7 and others say 6, so yeah...
The Volkswagen Golf Mk6 was produced from 2008 to 2013 in Europe and from 2009 to 2014 in the US. Your car is a 6th generation Golf.
As the Mk5 was still produced up to 2010 even after the introduction of the Mk6 in 2008 or 2009 dependant on your territory, the year alone is not enough to tell. Plus the car could be either late or early registered.
To be honest, the easiest way to know is to look at it compared to reference pictures.
and a Mk6:
It can't be Mk7 as that did not arrive until 2012.