Based on the answer in this question Can you use the pull effect of Thorn Whip if the enemy is within 5'? if a creature is unwillingly pushed/pulled/dropped or otherwise forced to end its turn in your space, what happens?
I can't find anything saying what happens. All I find is that you can't willingly end your turn in another creatures space.
What happens if you start your turn in the same space as another creature? Does anything happen to the creature that is forced to be in your space?
The space rules are mum about effects of starting a turn in the same square as another creature.
The creatures in the same space are uncomfortably close for combat. The moving around other creatures rules for combat state you can't willingly stay in that situation:
Whether a creature is a friend or an enemy, you can't willingly end your move in its space.
In combat, the characters are moving:
In combat, characters and monsters are in constant motion, often using movement and position to gain the upper hand.
The players can choose how much of their move to use, including 0:
You can use as much or as little of your speed as you like on your turn
The rules state you are in motion and you can't end your move still in the same space as another creature. So using your move to travel 0 feet (staying in place) is not an option. Something has to give. Either you have to move the other creature out of the space or move out of the space yourself.
The language used to describe features of tabaxi support moving 0 during a turn:
Once it uses this ability, the tabaxi can’t use it again until it moves 0 feet on one of its turns.
It could be construed that occupying the same square as another in combat involves restricting ones self in a similar fashion to squeezing through a restricted space. If this is reasonable for your table, both you and the other creature would have disadvantage on attack and dexterity rolls until the situation resolves.
There have already been quite a few questions on similar scenarios and the following are quotes from answers:
This answer to What happens when your movement is set to zero while attempting to move through a friendly space?:
[...] Even though Anne started her turn sharing a space, she cannot casually disregard it. She must move to another space, even if that would provoke an opportunity attack, trigger a known trap, or just be tactically disadvantageous [...]
This answer to What happens when you end up sharing an enemy (or friend's) space due to a size change?
[...] On your next turn (or Zorb's next turn, if it happens first), you must move to no longer be sharing a space (if it's possible) [...]
This answer to Does the rule that you cannot willingly end your move in another creature's space force or prevent certain actions?:
[...] The rules are unclear whether a player may receive their once-per-turn movement and not spend it to remain occupying the space, but this is clearly against the spirit of the rule. If a player has movement, I believe it should be spent to vacate the space [...]
This answer to Does the rule that you cannot willingly end your move in another creature's space force or prevent certain actions?:
[...] Keep in mind also it says movement, not turn. You can start and end your turn in another creature's space and only be required to leave it if you move and can get clear [...]
This answer to Can a Druid pull a target through other creatures with Thorn Whip?:
[...] Once one of the two creatures has a turn, it gets movement and is subject to the requirement that it must move out of the shared space.
This answer to If a creature starts their turn in another's space, are they forced to move out?
[...] Because they get to move on their turn and cannot willingly end their move in another creatures space, they must move out if able.
It's pretty clear that people don't agree on whether the rules require you to leave the space on your next turn or not. All we know is that the rules state this:
Whether a creature is a friend or an enemy, you can't willingly end your move in its space.
Does ending your turn counts as ending your move? Can you "end your move" on a turn where you haven't actually moved yet? It's unclear and not agreed upon.
Here are some comments that were left on those answers:
If Anne never take a move, how did she end her move?
I agree with your assessment; while it doesn't make that much sense, I don't think Anne is forced to move on her turn.
But if you don't start your move, can you end it?
[...] You always start your move every turn automatically - there's no Action or Bonus Action or Move Action (those don't even exist) to activate the ability to move each turn [...]
