Accessing Main Form controls from another Form

Question

Ok,
I've searched long and hard for this question, and I can't quite figure it out.  I'm new to C#, have a bit of background in VB/VB.NET, so bear with me...
I'm trying to start/stop a timer on my main form (Form1) from (let's call it) Form2. 
I have no problem changing properties on any other forms, ie: form3 control properties from form1 or 2).  I've run into a snag now that I'm trying to start/stop my timer which happens to be on the main form.
I've set the modifier on the timer control to public, but it won't show up.  From what I've read the main form isn't like other form objects.  I don't quite grasp that concept? This whole Class/Object Orientated programming thing is new.
All I want to do is start/stop the timer control on form1 from a button on form2.
Thanks.
Jeff.