Hi,
My application designed to allow user to have five product types. They share some common fields, with certain fields for each of proudct types.

Currently, I created productformbase and inherited from it with five specificproductform. However, I found it is not as user friendly if I changed something on base form. It caused I can't open my inherited form.

My question, do you recommend to use inherited form or template?

Thank you

Well there are number of things that you could do here.  In the example that you gave, however, I would probably create a user control (based off of the SF USer Control) that could then just be dropped on each form that needed this functionality.

You could create an inherited form, but there are a lot of limitations here...especially, as you learned, when you need to extend it further.