“未必的”常见的英文表达可以是 “not necessarily” 或 “not always the case” 。
“not necessarily” 较为常用和简洁，例如：That doesn't necessarily mean he's wrong.（那未必意味着他错了。）
“not always the case” 语气稍正式一些，例如：It's not always the case that the more expensive, the better.（并非总是越贵越好 。）