Day 21: Subjects and verbs must agree in number.

This is pretty simple. Verbs with subjects that can be replaced by he, she, or it require an “s.” Others don’t. Consider this sentence.

“Bob loves tuna sandwiches.”

“Bob” can be replaced by he, so “loves” needs that “s.”

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

This is a little more complicated with to be verbs. People often have problems choosing between the plural are and the singular is. Consider this sentence:

“Where this starts to cause problems IS when the subject and phrase before the verb ARE complicated, as in this sentence.”

We need the “is” because that verb is referring to “Where this starts to cause problems,” which can be replaced by it. We need the “are” because this verb is referring to both “subject” and “phrase,” which can be replaced by they.

To determine whether to use is or are, find the subject of the verb and determine what pronoun will replace it.