MOSTused to mention the main part or cause of something, the main reason for something etc 主要地，大部分地 SYN primarily
5 GRAMMAR 语法
Mainly is not used at the beginning of a sentence. mainly不用于句首
They play mainly on Wednesday evenings (NOT 不说 Mainly they play...).
他们主要是在周三晚上演奏。
n Grammar
You usually use mainly in the middle of a sentence:
They play mainly on Sundays.
People go there mainly because of the beautiful beaches.
You also use mainly at the end of a sentence, especially in conversation:
I read novels, mainly.
It is much less commonly used at the beginning of a sentence.
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In written English, people often prefer to use chiefly or primarily, which sound more formal than mainly: 在书面英语中，人们往往更喜欢用chiefly或primarily，这两个词听起来比mainly更为正式
The disease is caused chiefly by poor sanitation.
这种疾病主要是卫生条件差引起的。