そんなにSo (much), That much, Like that

そんなに, like そんな, is one of the most common ways to express that one thing is 'like' something else in casual conversation. The addition of the particle に just highlights そんな as being the 'destination' of the 'like' amount. This construction is regularly translated as 'so much', 'that much', or simply 'like that'.

As a word, そんなに is used adverbially. This means that it may come before any verb, い-Adjective, or な-Adjective.

Almost any そんなに based sentence can be translated literally as 'to an extent like (A)', where (A) is the word that comes after そんなに, and に indicates 'to'.

As そんなに is another example of こそあど言葉(ことば) (words like これ, それ, あれ, and どれ), it means that こんなに, あんなに, and どんなに may also be used.

Despite this, そんなに is almost considered a set phrase, and is therefore the most common out of all of the possibilities (except for どんなに, which is often used when explaining/giving reasons for something).