"Why are you so quiet today?"

Translation: 为什么你今天很安静？

[ShreckYe]

你今天为什么这么安静？

[Wengseng30]

This should be acceptable too.

[deactivated user]

Possibly. I know 什么 appears in the order you have suggested.

[KX3.]

It is accepted now.

[NasuSamaruk0]

Duolingo does not include subjective sentences like this, though I accept that as correct.

[RobinThor]

Why not 为什么你今天太安静? Where is the "so" in 为什么你今天很安静？?

[GuardianArcana]

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!

[ArMth1]

Is it acceptable to use 静 instead of 安静？

[KX3.]

Yes, you can say 你今天怎么那么静？

[kuyaC]

Can 你 今天 and 为什么 be put in any order?

[FHqXhJ]

Why is "so" translated as "很" here? It seems to me that "so" and "very" are only somehow similar in meaning, not completely the same, and each of them appears to have its Chinese equivalent ("很" vs. "这么"/"那么").