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I have seen two example of this.
The first example come from "恋する星座" part 9 where the girl said a sentences with words "行かなきゃ"
The second example come from the song "Catch A Break" sung by Eguchi Takuya. The lyrics goes "買いに行かなきゃ"
Why does they use "-なきゃ" in said examples? Is it used only in daily conversation? What are the other example of words using "-なきゃ"?
[Sorry if I ask too much I'm really curious]
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
-なきゃ is a common casual contraction (amongst a few other variations) for the form -なければいけません, which literally means “it is no good if the action is not done”, aka. showing obligation that the action must be performed.
from your example:
行かなきゃ: must go
買いに行かなきゃ: must go to buy
FYI, here are some other variations and contractions of the structure
Formal
なければいけません
なくてはいけません
なければなりません
なくてはなりません
ないといけません
Casual
なきゃ(いけません)
なくちゃ(いけません)
なきゃ(なりません)
なくちゃ(なりません)
source: my experience and my Japanese class notes
