horizontal nav vertical sub and responsive

Seeking a solution to the next:
Horizontal main nav.
When you click on the parent, only the childs of the active parent are shown in the left bar.
I found solutions for this with a plugin setup in a widget. But this is not working with a responsive menu. Then the childs are not visible in the responsive menu.
I also read a solution by using Conditional Tags. But I’m not a star in PHP.

Hope somebody can help me.