How do I create a sub-topic and a sub-sub-topic?

It’s not what the users want, but what in the earlier forum they evolved to use, unchaperoned. Very few, as I remember, went that far, but they could have done, any time one of them wanted to.

I will have to look into the use of tags. I’m just at the start of trying to learn how to get Discourse to do what I want.