Hi Amanda,
Thanks for your interest. Our plugin does work with IE8. Although we do our best to make things compatible with popular themes, once in a while themes will conflict with plugins. I recommend that you check out the compatibility of The Events Calendar with Definition by downloading our free calendar plugin (from https://theeventscalendar.com/wordpress-events-calendar/).
The Events Calendar Pro adds greater functionality and gives you access to our Pro support forums, but the core of the calendar is the same- in fact, the Pro plugin works in conjunction with the core plugin. In general, themes and other configurations that work nicely with the core plugin with also play well with the Pro plugin. That way, you can test things out risk free without spending any money.
If you have any questions or need further clarification, don’t hesitate to ask. You can add to this thread or start another topic here in Pre-Sales.
Cheers,
Leah