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]
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2019-03-28T07:26:43.543
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2019-03-28T08:30:30.040
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question
grammar
What's the use of "-なきゃ"?
-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
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answer