how to add forum in wordpress website without plugin ?

Basically, it's quite similar to the basic crud operation with a touch of authentication so that nobody could post unwanted contents on the forum.

You can add your own custom data table for forum questions, answers, likes, comments and users. This link How to add custom data tablein wordpress dashboard will guide you through the process on how to add, display, delete and edit data from a custom table in wordpress.

Rest is all logic.

Since, it's a pain to create all those tables and then write your own piece of logic for everything, most people use plugins like bbpress, buddypress, forumengine, etc.

If you still want to write it on your own and need further assistance let us know and we might come up with a detailed article on that as soon as possible.