Only played the game once, can any one help?

Can the keeper perform more than one action per turn? e.g. Can he use action cards and move monsters against an investigator in the same action phase?

As much Threat as you're willing to spend, you can. Some cards may limit you to one action per monster (Command Minion), so you can't move one monster 12 spaces with your Threat, but you can move six monsters two spaces each for example.

The only overall limit on the Keeper's actions is that during each investigator's turn the keeper can only play one Mythos card against that investigator.