I am English and I think this is difficult to answer. I would be surprised if something unexpected happened - for example if the seeds I had planted had sprouted by the next day or if someone jumped out and said "Boo" unexpectedly. I would be amazed if the thing that happen was a mixture of very surprising, exciting and confusing - I was amazed when the magician made a coin appear from nowhere. Or I was amazed that he would risk everything to achieve his goal. Or that is an amazing painting you have done. To be taken aback implies you are a bit upset by the surprising thing that happened e. g. I was taken aback when she burst into tears after I gave her a present. I think astounded is a stronger version of taken aback - you might step away, stare and open your mouth (your jaw would drop) if you were astounded.

Oo this is a tricky one. Part 1 is easy. Surprised - amazed - astounded. They mean the same thing, but in increasing order of intensity. Amazed = very surprised; astounded = even more than amazed. 2. "Taken aback" is harder and I've really had to think about it. It means "surprised" (it's not very strong) but is only used in certain contexts: A. It's only a negative emotion. You can be pleasantly surprised, but you can't be pleasantly taken aback. B. You are only taken aback, I think, by things directed personally at you. So I was surprised at the price of the car. I was taken aback by the salesman's rudeness. (This implies that his rudeness was directed at me) There may be more restrictions on the use of "taken aback" which I haven't thought of and which other people will come up with.