For a course with several cohorts and one forum, it is possible to set the forum to have a separate forum space for each cohort group.

Go to the forum administration menu and expand the ‘common module settings’ dropdown. For group mode, select ‘separate groups’ (this means each group member can only see their own group, other groups are invisible). Click on Save and display.

You can use the group function for other course activities such as Choice (poll), Wiki, Glossary, Blog.