The following sentence, along with its translation, is from the Dictionary of Japanese Grammar (intermediate).
吉田さんは日本人だ。しかも、小説家だ。だからと言って、日本語が教えられるとは限らない。 (Ms. Yoshida is Japanese. And she is a novelist. But she may not necessarily be able to teach Japanese.)
My problem is at the end of the sentence. The way I parse it goes like this (just a rough translation to help the discussion):
日本語が教えられる = Japanese that is "teachable"
日本語が教えられるとは = About Japanese that is "teachable"..
日本語が教えられるとは限らない = About Japanese that is "teachable", it is not limited.
However, this disagrees with the translation. Why does 限らない here still mean "limited" or "restricted", even though it is negative? And how would the sentence change if it were to end with just "限る"?