What it means
とは限らない expresses that a statement is not necessarily true or does not apply in every case. It is used when rejecting an assumption without completely denying the possibility. It commonly follows a plain verb or i-adjective, and follows a noun or na-adjective with だ. The polite counterpart is とは限りません.
Usage note
とは限らない means that something is not always or necessarily the case; it does not mean that the statement is definitely false. Do not confuse it with わけではない, which usually denies a particular interpretation or implication rather than the general certainty of a statement.
How to use it
彼が専門家だとは限りません。
He is not necessarily an expert.
この部屋が静かだとは限りません。
This room is not necessarily quiet.
この時計が高いとは限りません。
This watch is not necessarily expensive.
田中さんが来るとは限りません。
Tanaka will not necessarily come.
彼が専門家だとは限らない。
He is not necessarily an expert.
この部屋が静かだとは限らない。
This room is not necessarily quiet.
この時計が高いとは限らない。
This watch is not necessarily expensive.
田中さんが来るとは限らない。
Tanaka will not necessarily come.