Are you attempting to isolate a portion of a clip, using the red [ and blue ] brackets at the ends of the preview window time line?
If so, first click on the clip in the media window (upper left). Then move the scrubber (red line) to where the clip is to begin. Click the START button.
Move the scrubber to the spot where the clip is to end and click the END button.
Now use the green arrow that's in the tool bar just above the media files to place the clip. Click the chevron to the right of the arrow to see the options.
Follow this procedure every time another section of the same clip is to be utilized.