Double negative not making sense


From NHK Easy Japanese News:

>
> 「民主主義｛みんしゅしゅぎ｝は、一度｛いちど｝運動｛うんどう｝を止｛と｝めると、使｛つか｝わない筋肉｛きんにく｝のように弱｛よわ｝くなってしまうので、運動｛うんどう｝を続｛つづ｝けなければなりません」と言｛い｝いました。

If I translate literally:

> "About democracy, if stop exercising once, becomes frail like unused muscle
> therefore, if don't continue exercising don't become," she said.

The last part, logically, seems to be backwards. I would expect either of

> ～運動を続けなかればなります。
>
> ～運動を続ければなりません。

Is the original sentence correct and so what is its meaning and why?

I am familiar with "must do" expressions using two potential negative forms
like:

> ～なければいけません

But here the ending verb is simply なりません, not a potential form. Would this
also express "must do"?




Yes, ～なければなりません also means the same as ～なければいけません. These two can be regarded
as fixed phrases.

Therefore 運動を続けなければなりません means "they must continue the campaign."

It's enough if you understand this as a fixed phrase, but this can also be
explained in a literal way:

The verb なる can mean "to succeed" or "to complete" as described in
[jisho.org](http://jisho.org/word/%E6%88%90%E3%82%8B) and ～なければ means "if one
doesn't ～".

So ～なければなりません can be interpreted as "if one doesn't ～, it's not ok".




Yes, ～なければなりません also means the same as ～なければいけません. These two can be regarded
as fixed phrases.

Therefore 運動を続けなければなりません means "they must continue the campaign."

It's enough if you understand this as a fixed phrase, but this can also be
explained in a literal way:

The verb なる can mean "to succeed" or "to complete" as described in
[jisho.org](http://jisho.org/word/%E6%88%90%E3%82%8B) and ～なければ means "if one
doesn't ～".

So ～なければなりません can be interpreted as "if one doesn't ～, it's not ok".

