* CAN is used to talk abilities in the present time.
I can play football. You can play chess. You can ride a bike. (Sé jugar a fútbol. Sabes jugar al ajedrez. Sabes montar en bici.)
CAN'T is the negative form.
* COULD is used to talk abilities in the past time.
I could ride a bike. (Sabía ir en bici)
COULDN'T is the negative form.
*MUST is used to talk about an obligation. You must listen to the teacher.(Debes escuchar a tu profesor). You must study hard. (Debes estudiar duro).
*MUSTN'T is used to talk about prohibition. You mustn't shout in the church. (no debes chillar en la iglesia)
