With the video opened, you can control the playback with the controls right below the video itself.
The "Stop" button (with a "Stop" symbol) will stop the playback and return the timeline cursor to the initial position (at 0.0 second).
The "Play/Pause" button will play the video. This button will change it's symbol according to the actual playback state. If it is playing, it will show a "Pause" symbol. So if you click that button while the video is playing, it will pause the video. If the video is stopped or paused, the symbol is the "Play" symbol.
The button on the left side of the "Stop" button is a shortcut to play from the last subtitle beginning.
The button on the right side of the "Play/Pause" button is a shortcut to play from the next subtitle beginning.
The following is a short video showing how to use that playback controls.