I was simply taken ABACK? = I was simply astonished, amazed, shocked, surprised by his behavior. Consider please, I am not a native speaker.

The correct form is "be taken aback" Emma was somewhat taken aback by his directness. The news really took us aback. All of us were a bit taken aback to learn that John was moving to England next month. I must admit that I was taken aback when I heard we weren't receiving our bonuses this year.

Ali is correct; it's to be taken *aback* if you mean that you were shocked or surprised.

An example might be if someone told you news that really surprised you. For example: I was very taken back when Sam told me that he hated my family. It shows surprise. It is like someone told you something that was so surprising that you stopped and gasped at the surprise of the thing that person told you. Additionally, if someone told you something harsh about yourself you might say that you are taken back. Ex: I was very taken back when Allison said that about me.