When you talk about the forties, you are referring to numbers between 40 and 49. For example, if you are in your forties, you are aged between 40 and 49. If the temperature is in the forties, the temperature is between 40 and 49 degrees.
例句
He was a big man in his forties, smartly dressed in a suit and tie.他是一個(gè)四十多歲的高大男子，穿西裝打領(lǐng)帶，看上去很精神。
N-PLURAL
二十世紀(jì)四十年代(指從 1940 年至 1949 年)
The forties is the decade between 1940 and 1949.
例句
Steel cans were introduced sometime during the forties.鋼罐是 20 世紀(jì) 40 年代時(shí)開(kāi)始使用的。