In downtown Los Angeles,it's going to be sunny and clear tomorrow morning.
In the early afternoon,it's going to be windy.
In the late afternoon,it's going to be cloudy這三個(gè)句子都是說(shuō)天氣情況的,sun、wind、cloud、都是名詞,不可以直接跟在it's going to 后,所以要借用be動(dòng)詞,be就是動(dòng)詞原形,這樣才符合語(yǔ)法.
In the early evening,it's going to rain in downtown Los Angeles.
But in the mountains,it's going to snow.這兩個(gè)句子雖然也是說(shuō)天氣,但是rain、snow都可以作動(dòng)詞使用,故不用借助be動(dòng)詞.