A friend asked the other day about how to handle t when her two year old takes something from her younger sister and then when told to give it back seems to do exactly the opposite of whatever she was told to do. Surprise!
Here are some basic facts for dealing with two year olds.
If you say "No" or "Don't" then all they hear is stop. But not stop doing that. They hear stop moving or freeze. So they wil stop moving for a brief time and then they will go right ahead with what they wee doing because they don't have the ability to think in terms of reversing their actions. When you say something simple like, "Don't take the baby's toy." it is asking the child to understand that don't means something like the reverse of what follows. Any word that they don't know what to do with they lop off of the sentence ans so what youa re left with is, "Take the baby's toy."
Another thing is that they love it when you play games with them and they like the interaction. The problem is that they don't know how to initiate the interactions. So if they take something and it illicits a response from you, they think, "oh, I got their attention, I will now do >>>> and we will be playing." This is just a part of development so you have to consider where they are and work them gradually into your world.
Two-year-olds also get centered on objects. So once it is in their hand they have a hard time separating what you are saying from what they are wanting. This means you have to physically get engaged and not just verbally. It also helps if you help them to do a positive thing like handing them something to give to the younger child. Model what you want rather than demanding what you don't want to stop.