in the 1950s 不加the为什么是错的?in the 1950s 和 in 1950s 那个错那个对啊?有什么区别吗?

in the 1950s 不加the为什么是错的?in the 1950s 和 in 1950s 那个错那个对啊?有什么区别吗? I was born in _____andI was born in ____and studied at this school ____.A in 1980; in the 1990B in the 1980；at the 1990sC in1980; at the 1990sD in 1980； in the 1990s我知道是选D,但是为什么呢?In 加年,不加介词吗?in the 1990s 为什 The policemen are in uniform为什么uniform不加s The cloud come from the water in the rivercome 为什么不加s不是第三人称吗 Trees are planted in most of the mountain v__ village加s 为什吗 Don't arrive late for class.Don't eat in the classrooms.class,classroom,同为名词,为什么class后面不加S,classroom加S? the best things in life are happy中的thing 加还是不加S,为什么? in the mountain(能加s吗?) 是不是she's liveing in the hotel.里的she加s（包括he,they,we和其他名字结尾不是s的）,而以结尾为s的不加's? the government protection为什么不加's 为什么希腊人 the Greek 不加s i think he's still in ()bed,but he might just be in (the )bathroom为什么前面不加the,后边就一定要加the come in first为什么不加the famous in history 为什么不加the 为什么in Beihai Park 不加the look sb in the eye中eye为什么不加s why not put some lemon in the tea?lemon 为什么不加S He eats in the classrooms为什么翻译中eat不加s