The subscribe box behaviour is ergonomically wrong (see attached screenshots) and semantically confusing. Indeed, the checkbox is shown as already ticked, whereas in fact subscription is off!
There are two ways to correct it:
(a) put a checkbox with the text "subscribed to this forum" whereby the checkbox shows a tickmark if the condition is true and none if false;
(b) put a button whose text indicates what will happen when the button is pressed, i.e. the button legend is "subscribe to forum" when the user is not subscribed, and "unsubscribe from forum" when the user has a subscription.