Yes you can but moment you create sub site you're site is not modern site it is just like classic site with modern site because thumb rule for modern site is it should not have subsite because this is connected to office365 group.
If you mean use modern site templates (modern team sites and communication sites), the answer is No...but as stated by Ravi, you can create a subsite using a classic template and you will get your modern stuff there: modern pages, modern WebParts, modern lists and libraries, ...
Yes you can but moment you create sub site you're site is not modern site it is just like classic site with modern site because thumb rule for modern site is it should not have subsite because this is connected to office365 group.