Click your Avatar icon near the top right corner and choose Preferences.
On the preferences page you can choose how you get notified. A common solution is to turn on "Everything in my Inbox page." Check out the other options, and set them up as you prefer (these are Preferences after all). Example, maybe you want to "Include body of content in email."
Once that is set up, you will get email notifications.
When you participate in an individual topic, you automatically follow it. So you'll get email if someone replies.
If you want notification for any activity in a forum, do this.
- Go to the forum
- In the upper right, click Follow
- Pick Inbox
- Click Done
Email alerts are identifiable as coming from the community, so you should be able to filter them as you wish.