slow down
With or without an object
Go slower, or make something go slower
Slow down, you are driving too fast.
With or without an object
It can be used with no object, and it can also take one. With an object it is usually separable: turn the volume up, turn it up.
The particle: down
This verb belongs to the group ‘getting less’.
Practise it
Add it to your daily practice or see every verb with down.
Further reading: slow down on Wiktionary (CC BY-SA).