wpForo has built-in forum/topic subscription with double opt-in subscription confirmation function.
Subscribe for new topics – This allows you subscribe to new topics of certain forum.
If the double opt-in subscription system is enabled you’ll be asked to confirm your subscription via email.
User clicks on “Confirm my subscription” link and starts getting new topic notification emails:
User can unsubscribe to the forum whenever he/she wants by clicking on Unsubscribe link.
Also you can unsubscribe using the Unsubscribe button located under the Forum title you’ve subscribed.
This is similar to Forum subscription. The only difference in Subscription buttons. You can find topic subscription button in topic page. Click on a topic title/link and find [Subscribe for new replies] link.
Another easy way to subscribe to own topic or to other topics is using Subscribe to this topic checkbox.