Although it is not considered to be one of the primary 18 助動詞（じょどうし） (auxiliary verbs) that are used in modern Japanese, ず is a classical auxiliary verb that has the same role as ぬ, and ない. This role is negating the verb that it is attached to. ずに is simply the addition of the 格助詞（かくじょし） (case marking particle) に to ず. Together, they mean ‘without doing’, in a similar way to ないで.
To use ずに, attach it to the 未然形（みぜんけい） (indeterminate form) of the verb that you wish to express as ‘without doing (A)’. This is the same form used with ない.
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朝（あさ）ご飯（はん）を食（た）べずに仕事（しごと）に行（い）ったから、仕事（しごと）中（ちゅう）に倒（たお）れた。
Because I went to work without eating breakfast, I fainted in the middle of my job.
水（みず）を飲（の）まずに運動（うんどう）をしていたから、頭（あたま）が痛（いた）い。
Because I exercised without drinking water, I have a headache.
ずに is quite a bit more formal than ないで, but still frequently used in modern Japanese. Especially as part of set phrases, like ずにすむ (which we will learn about in another lesson).
When used with a
する verb,
する will become せずに, not しずに. Both せ and し are 未然形（みぜんけい） forms of
する. し, the more common form that is used in east Japan appears with ない.
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勉強（べんきょう）せずにテストを受（う）けたから、テストの点数（てんすう）が低（ひく）かった。
Because I took the test without studying, I got a low score.
あんまり無理（むり）をせずに頑張（がんば）ってくださいね。
Please do your best without trying too hard.