"Why are you so quiet today?"
I had to look this up because I still have trouble with it.
Summary: 是 cannot be used to connect a noun and adjective, 很 is what is used instead.
X: 你是高。You are tall. ✓: 你很高。You are tall.
很 and 非常 both mean very
✓: 你很高。You are tall. ✓: 你非常高。You very are tall.
太 refers to the greatest of something, or is usually used with 了to express to much of something.
✓: 你很高。You are tall. ✓: 你太高了。You very are tall.
I hope this helps you like it helped me!