必ずしも means "not necessarily": it denies that something is always true and leaves room for exceptions. It is an adverb that needs a negative at the end of the sentence, most often or .
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Expensive things aren't necessarily good.
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Not everyone necessarily agrees.
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What he says isn't always right.
必ずしも or 必ず
on its own means "certainly, without fail" and goes with a positive. Add しも and the sentence turns negative: "not always". 必ずしも never stands with a positive ending.
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If you work hard, you will succeed.
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Working hard doesn't necessarily mean you'll succeed.
とは限らない and わけではない
says "it doesn't follow that".
says "that's not to say", and often comes after
, "just because". Before either, a noun or a な-adjective can take だ:
.
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Rich people aren't necessarily happy.
、。
Just because something is cheap doesn't mean it's poor quality.
Lingaku