What happens when a character is hit with both haste and slow?
— Paul (@P_Dubs) June 26, 2018
If two different spells are successfully cast on you, you are now under the effect of those spells. #DnD https://t.co/J754sJRw0N
— Jeremy Crawford (@JeremyECrawford) June 26, 2018
So that would be…
The +2 AC and -2 AC cancel out.
You suffer a -2 penalty on Dexterity saves, but you make them with Advantage.
You can use either an action or a bonus action, but not both.
You gain an additional action on each turn that you can use to make a single weapon attack, dash, disengage, hide, or use an object.
You also can’t make more than one attack during your turn despite items or abilities – so if you attack with your 1st action, you can’t use the second action from Haste to attack again.
And if you cast a spell with a time of 1 action, you roll a d20 and if you get 11+ then you have to spend your next action continuing to actually finish and cast the spell before it actually is able to work, and if you can’t you lose the spell.