If someone is restrained by the entangle spell making their speed zero, can I knock them back with spells like thunderwave or thunderous smite?
You can also shove your allies out of the area of effect (and they can opt to not resist the shove). So any forced movement form works.
The only effect of being restrained is to have move set to zero. If the spell description stated that the creature cannot be moved, then it wouldn't, but that is not the case in most situations.
The above is in line with the line of ruling from 5e where spells do what they say, and not what they don't say.
When an effect force-moves the creature, it does not spend their movement allowance, and doesn't cause Attacks of Opportunity also.
The status that set move to zero would only end if the creature is moved outside the effect.
For a grapple, it is the reach of the grappler.
A creature's movement speed only affects its ability to move, not outside forces abilities to move it. If the spell only affects the creature's movement speed, then it only affects the movement speed. If it makes it incapable of moving or being moved, then it can't be moved.
There is a lot of good evidence of this, but the best I can find is in the description of the Grappled condition.
- A grappled creature’s speed becomes 0, and it can't benefit from any bonus to its speed.
- The condition ends if the grappler is incapacitated (see the condition).
- The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.-PHB p.290 (bold added)
Thus, we have RAW support for the idea that it is indeed possible to force movement on something whose speed is 0.