In everyday Japanese, そんなに is a frequently used adverb to indicate that something is a certain way 'to that extent' or 'to that degree'. It's related to そんな, but the addition of the particle に allows it to modify verbs and adjectives. Common English translations include 'so much', 'that much', or simply 'that' (as in 'that difficult').

Because そんなに functions as an adverb, it is always placed directly before the verb, い-Adjective, or な-Adjective that it describes.

• そんなに飲(の)んだら、気分(きぶん)が悪(わる)くなるよ。
  If you drink that much, you're going to feel sick.
• そんなに高(たか)いと思(おも)うなら、買(か)わなくてもいい。
  If you feel it's that expensive, you don't have to buy it.
• そんなに上手(じょうず)じゃないなら、もっと練習(れんしゅう)したほうがいい。
  If you aren't that skilled, you should probably practice more.

You can understand this grammar more literally by thinking of に as the particle 'to'. Therefore, そんなに can mean 'to that extent' or 'to that degree', where 'that' refers to something known to the listener.

そんなに is part of the こそあど言葉(ことば) series (words like これ, それ, あれ, どれ). This means that こんなに (this much), あんなに (that much over there), and どんなに (how much) are also used in the same way.

• こんなにおいしいケーキは食(た)べたことがない！
  I've never had a cake this delicious!
• あんなに勉強(べんきょう)したのに、もうゲームをしているの？
  He was studying that hard, and he's already playing games?
• どんなに彼(かれ)のことが好(す)きでも、彼(かれ)は同(おな)じ気持(きも)ちじゃないと思(おも)う。
  No matter how much I like him, I don't think he feels the same way.

Of all these forms, そんなに is by far the most common in general conversation. However, どんなに is also frequently used, especially in the pattern 「どんなに〜ても」 to mean 'no matter how...'.