Modal & direct frequency analysis

From the frequency response analysis of a flat plate, I have couples of questions:

1. why do we only constrain dof3?

2. is there a specific direction of the dynamic loaded is applied? when I view my result, it seems like the direction of deflection is mainly in Y- axis. 

3. If I'm doing a frequency response analysis on an face of a optimize shape, can I follow the tutorial to perform the same analysis?

4. I also applied Modal and Direct analysis on the same model, but they provide me different result. Can you explain why is that?