, 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
If you eat that much, you are going to break your stomach. (Natural Japanese for ‘stomach ache’)
If you think it’s that hot, you should turn on the air conditioner.
If you dislike it that much, you don't have to force yourself to eat it.
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
is another example of こそあど言葉（ことば） (words like これ
, and どれ
), it means that こんなに
, and どんなに
may also be used.
Is it okay to receive this much? (Are you really giving me all of this?)
You are still hungry, even after eating that much?
No matter how hard I try, I don't think that I can become like my senpai.
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).