Question

Hoy no __________ Domingo.
a) es
b) son
c) soy
d) somos

Answer 1

The sentence is trying to say "Today is not Sunday."

We can use a process of elimination, to solve this blank.

"Son" means "they are."
That wouldn't make sense.

"Soy" means "I am."
That wouldn't make sense either.

"Somos" means "we are."
That wouldn't make sense at all.

And so, we're left with just one: "es."
"Es" means "It is."

So, we say "Hoy no ES Domingo."

"Today IS not Sunday."

Answer 2

Thank you so much for explaining it to me. c: