One of the most basic mistake singers make when they start singing is the placement of their tongue.   The tongue is the largest muscle in the vocal tract and to ensure that it does not obstruct the flow of the air as the singer aspirates (releases the air flow), or blocks  the throat, the tongue should always be placed against the lower back teeth when a singer is singing.

Picture of mouth, a singer must be aware of the position the tongue, soft palate and hard palate when singing