Here’s an exercise that will help you to grasp the difference between the vowels OU and U.
For students of French as a foreign language
The vowel written U in French is not found in as many languages as the vowel written OU. The latter is pronounced like OO in English. Both vowels are round vowels (i.e. the lips are rounded). The difference lies in the position of the bulk of the tongue: toward the back of the mouth for OU, toward the front for U.
Again, my advice to you is to try to repeat all the minimal pairs of this exercise at least twice, and if necessary you could try also to practice again the exercise on I and U.