If you say that something is not necessarily the case, you mean that it may not be the case or is not always the case.
Anger is not necessarily the most useful or acceptable reaction to such events.
对待此类事件,生气未必是最有用或最适合的反应。
If you say that something necessarily happens or is the case, you mean that it has to happen or be the case and cannot be any different.
Brookman & Langdon were said to manufacture the most desirable pens and these necessarily commanded astonishingly high prices.
据说布鲁克曼兰登公司制造的钢笔最令人满意,而这必然导致高得惊人的价格。
not necessarily
If you reply 'Not necessarily', you mean that what has just been said or suggested may not be true.
'He was lying, of course.'—'Not necessarily.'
“他当然是在撒谎。” “不一定。”