"Decide yourself" and "You decide" should be accepted. Besides, this shouldn't be here in the 1st place as it is the Imperative.
Well, I'm not sure, but it sounds right to me, and Google Search returns a few examples...
It did seem like the imperative, so I just put "Decide." That's the simplest translation (a command to someone else to decide), but it wasn't accepted.
I would say because the translation of "Decide" would be "Decidi". Simply "Decide" sounds a bit too authoritative to me.
Because "Decidi tu" is the imperative, and "Tu decidi" is the Present Tense.
Not necessarily. Since the form is identical, there is possibility of the present tense, especially since we are in the pretense section of the grammar lessons. It's difficult to hear because of the nuance. Placing the personal pronoun last emphasizes the subject of the verb.
There is a common overlapping of imperatives with the personal pronoun at the end. However, this isn't a hard and fast rule; they simply overlap.
For instance, you ask me: "who makes the decision?" I respond, "you make the decision." In English, I have to inflect my voice to emphasize you as the person who makes the decision. In Italian, I would respond, "decidi tu." In this scenario, it would sound funny, though still grammatically proper, to say "tu decidi." However, I'm still not giving an imperative sense.