Hi Lara,
I still get confused by this too, but one thing I try to do is ask myself a question while studying a new verb. "What?"
For example:
I went to the store.( Mein dukan gayi/a) Vs. I ate a sandwhich. (Mein ne sandwhich khaya)
In the first sentence if you ask "I went...what? It doesn't make sense because the subject isn't acting on anything. You went to a *place*.
In the second example you are acting on something. "I ate....what? (A sandwich).
Like I said, this is still tricky for me too, but I thought I'd share in case it helped. Best of luck!
-Ashley