For the Complete and Translate exercises (easy level), you must confirm your answers (and therefore the complete exercise) by pressing Ok. You can then press the circular green arrow to restart the exercise.
For the Translate exercises (difficult level) and Speak exercises, you must also confirm your answers (and therefore the complete exercise) by pressing Ok; but you must then answer the self-assessment question (Is your answer correct?) for all phrases of the exercise.
Once you have done this, a pop-up will appear: Assessment completed?, to which you must answer Yes or No.
If you reply Yes, your score will be displayed and you can then access the circular green arrow button, which enables you to retake the exercise if you wish.