How to design a Notification, Activity table for project management?

I'm going to create a simple web app for project management similar to trello. My database will contains:

• User
• Project
• Category (To categorize task)
• Task (include many subtask, comment, attachment)
• Subtask
• Attachment
• Comment
• Notification (Notify to users when something's changed related to them)
• Activity (To show all activities that users have done to a task)

Notification table that stored information when some one did a change to anything related to user, such as:

• moved Task from category to another category
• commented on a task
• attached a file on a task
• ...

And Activity table stored logs like notification table but just related to a task:

• moved task
• comment
• add/remove subtask
• mark subtask completed/incompleted
• ...

I don't know how to design 2 tables Notification and Activity, so I posted here for advices. Thanks!!!