The words pero and sino both translate to “but.” However, they cannot be used interchangeably.
Pero is used when you are looking to convey “nevertheless” or “yet.”
Examples:
Estaba muy cansada, pero no me dormí. (I didn’t fall asleep, but I was very tired.)
No tenemos gaseosa, pero tenemos limonada. (We don’t have soda, but we have lemonade.)
Ella no mira mucha televisión, pero siempre mira su telenovela favorita. (She doesn’t watch much television, but she always watches her favorite soap opera.)
Sino/Sino que is used when you wish to convey “on the contrary,” “instead,” or “but rather.”
Examples:
No nos sentamos en sillas, sino en el piso. (We didn’t sit on the chairs but on the floor.)
No estaban en una ciudad grande, sino en un pueblo pequeño. (They weren’t in a big city, but a small town.)
El libro no es de María, sino que de Marta. (The book isn’t Maria’s, but rather Marta’s.)
No hizo la tarea, sino que miró televisión. (He didn’t do the homework, but rather watched television.) There is also a more informal way to express this idea as sino que comes across as formal: Miró la TV en vez de hacer la tarea.
The easy way to remember the difference is that pero is sort of the basic “but” used in simple sentences. Sino/Sino que is for when you are offering an almost opposing fact or situation. Pero is more of a sentence break, while sino/sino que shows a contrast.
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