When performing squats or leg press exercises don’t let your knees go in front of your toes, this can potentially cause injury to your knees. When squatting or leg pressing, you want to ensure that your legs come to a 90 degree angle with the floor (or platform if leg pressing) and that your knees do not go in front of your toes. To ensure you get the correct form down, make the weight lighter to practice the form before getting into heavier sets.
If during bench press you arch your back, this can potentially cause significant injury to your back. If you have to arch your back to lift the weight up, that means you should lower the weight. Once back on the bench, try to tell your body to focus on pushing your lower back into the bench as you lift the barbell up. This will help prevent arching. Another common form mistake that I frequently witness in the gym during bench press is people lifting their heads and necks off the bench as they press up with either dumbbells, or barbell. This can strain and give very painful injuries to your neck and spine. Your head should remain on the bench as you press up.
While performing bicep curls, if you have to use momentum to get the weight up (as stated earlier), this creates a lot of stress on the shoulder girdle and if the weight is too heavy it can knock you off balance. You would benefit more by reducing the weight and concentrating the movement. In order to prevent momentum, the best suggestions are to perform bicep curls while sitting on a bench, or with your back pressed against a wall. This will help to keep the movements strict and concentrated to your biceps as intended.
To sum it up, it is by far a much better idea to reduce the weight in order to prevent injury and to ensure that you are working the intended muscle that is supposed to be worked while performing exercises. If you have a beginner, always start them out with a beginner workload and with persistence, dedication and will-power, they will be able to lift that heavy weight in no time!