主要 + Verb + 在 + Place / 主要 + Noun
主要 (zhǔyào) means "mainly," "primarily," or "major" and is used to highlight the most important element or the principal aspect of something. As an adverb, it modifies verbs to indicate that an action primarily occurs in a certain way or place. As an adjective, it modifies nouns to mean "main" or "primary," as in 主要原因 (main reason).
我的工作主要是负责客户服务。
Wǒ de gōngzuò zhǔyào shì fùzé kèhù fúwù.
My job is mainly responsible for customer service.
这家店的顾客主要是年轻人。
Zhè jiā diàn de gùkè zhǔyào shì niánqīngrén.
The customers of this store are mainly young people.
空气污染的主要原因是工业排放和汽车尾气。
Kōngqì wūrǎn de zhǔyào yuányīn shì gōngyè páifàng hé qìchē wěiqì.
The main causes of air pollution are industrial emissions and vehicle exhaust.