~なけりゃ or ~なきゃ, how to know which one to use?


> 風邪を引いたから薬を早く飲まなきゃ

I know that generally, `eba` contracts to `ya`  
(答えれば → 答えりゃ）  
(おけば → おきゃ）

If the original form in this sentence is 飲まな[ければ]{ll}ならない.いけない  
how shall we choose between 飲まなきゃ and 飲まなけりゃ?




It is all about the **_degrees of formality/informality_**.

The plain fact is that 「なきゃ」 sounds more informal than 「なけりゃ」 and 「なけりゃ」 is
more informal than the "dictionary form" 「なければ」.

There are, however, no written rules regarding when to use any of those forms.
The unwritten "rule" would be that you manintain a certain level of formality
throughout what you say or write in one conversation or piece of writing. Just
like in English, you do not randomly mix multiple levels of formality.

Your sentence:

> 「風邪{かぜ}を引{ひ}いた **から** 薬{くすり}を早{はや}く飲{の}ま **なきゃ** 。」

reads okay (if I pretend to not notice the less-than-perfect word order in
「薬を早く」 ). 「なきゃ」 fits well because 「から」 is fairly informal as well (compared to
「ので」) . It is all about the balance. In the order of formality, one might tend
to say:

・「カゼを引いた **ので** 、早く薬を飲ま **なければなりません** 。」

・「カゼを引いた（ **ので/から** ）、早く薬を飲ま **なけりゃならない** 。」

・「カゼを引いた **から** 、早く薬を飲ま **なきゃなんない** 。」 Notice the 「な **ん** ない」.

In short, if you change one word, you will usually need to change some others
as well so the good balance of formality/informality will be kept within the
sentence.

For the fairness sake, though, one thing that I could not go without
mentioning is that in the real Japanese-speaking world, **「なきゃ」 is used far
more often than 「なけりゃ」**. It is hard to say 「なけりゃ」 - or is it just me?




It is all about the **_degrees of formality/informality_**.

The plain fact is that 「なきゃ」 sounds more informal than 「なけりゃ」 and 「なけりゃ」 is
more informal than the "dictionary form" 「なければ」.

There are, however, no written rules regarding when to use any of those forms.
The unwritten "rule" would be that you manintain a certain level of formality
throughout what you say or write in one conversation or piece of writing. Just
like in English, you do not randomly mix multiple levels of formality.

Your sentence:

> 「風邪{かぜ}を引{ひ}いた **から** 薬{くすり}を早{はや}く飲{の}ま **なきゃ** 。」

reads okay (if I pretend to not notice the less-than-perfect word order in
「薬を早く」 ). 「なきゃ」 fits well because 「から」 is fairly informal as well (compared to
「ので」) . It is all about the balance. In the order of formality, one might tend
to say:

・「カゼを引いた **ので** 、早く薬を飲ま **なければなりません** 。」

・「カゼを引いた（ **ので/から** ）、早く薬を飲ま **なけりゃならない** 。」

・「カゼを引いた **から** 、早く薬を飲ま **なきゃなんない** 。」 Notice the 「な **ん** ない」.

In short, if you change one word, you will usually need to change some others
as well so the good balance of formality/informality will be kept within the
sentence.

For the fairness sake, though, one thing that I could not go without
mentioning is that in the real Japanese-speaking world, **「なきゃ」 is used far
more often than 「なけりゃ」**. It is hard to say 「なけりゃ」 - or is it just me?

