Hi, chikitin,
You can use the J, K, and L keys for scrolling back and forth along the timeline. Tap L to move forward. Repeatedly tapping will speed up the forward scroll with each tap. Tap K to stop the scrolling. Tap J to scroll in reverse. Again, repeated tapping will speed up the backward scroll.
You can also use the left and right arrows on the lower right of the keyboard to advance or reverse the playhead one frame at a time. The up and down arrows advance or reverse one clip at a time.
The \ key sends the playhead to the beginning of the movie and starts playing.
The Modify menu item has selections for slow motion, fast forward, and rewind that lets you jump around in the timeline.
Click on the iMovie Help menu item and then click on Keyboard Shortcuts for a list of the iMovie shortcuts. You can also see the keyboard short cut or each function by clicking on any menu function item and the shortcut symbols will be shown in the drop down next to each function. You can also control-click on any video clip and a drop down will appear showing some basic funtion options with the Keyboard symbol next to the function.
-- Rich