[Michigan90]

Originally Posted by DragonShade

Thanks, now I know the meaning and function of this form

but I wonder what's the rule for this, I mean how to change other verb into this form?

ず is the continuative form (連用形 ren'yōkei) of ぬ, which is the conjugable auxiliary verb (助動詞 jodōshi) used for negation. Any verb before ず turns into imperfective form (未然形 mizenkei). Modern Japanese has three regular conjugation groups as well as two irregular verbs.

• 5-row group (五段活用 godan katsuyou): Imperfective form ends with -a.
• upper 1-row group (上一段活用 kami ichidan katsuyou): Imperfective form ends with -i
• lower 1-row group (下一段活用 shimo ichidan katsuyou): Imperfective form ends with -e

The irregular verbs are -する and -来る, which changes to -せず and -来(こ)ず.