To say that you do something with someone, Chinese uses 跟 (gēn) + person + 一起 (yìqǐ) + verb. The key point is that the whole "with someone" part comes before the verb, not at the end of the sentence as in English.
Subject + 跟 + person + 一起 + verb (+ object).
跟 means "with" and introduces the other person. 一起 means "together" and comes straight after the person, just before the verb. Time words such as 每天 or 昨天 go at the start of the sentence or right after the subject.
The most common mistake is to follow English word order and put "with my friend" or "together" at the end: 我去商店跟朋友一起 and 我们吃饭一起 are both wrong. In Chinese, 跟 + person + 一起 always comes before the verb.
Words like 想, 要 and 会 go before 跟, so the order is subject + 想 + 跟 + person + 一起 + verb.
If the subject already includes everyone, such as 我们 or 你们, you don't need 跟: just put 一起 before the verb. 我们一起去吧 is the natural way to say "let's go together".
和 (hé) can replace 跟 in this pattern with the same meaning; 跟 is a little more common in everyday speech. Note that 我和哥哥 can also simply mean "my brother and I".
To say you don't or didn't do something with someone, put 不 or 没有 before 跟, not before the verb: 我不跟他一起去, not 我跟他一起不去. Note the tones: 不 stays bù before 跟 (first tone), and 一 in 一起 is yì because 起 is third tone.
To ask who someone is doing something with, put 谁 after 跟. To invite someone, use 想 or 要 with 吗, or end a suggestion with 吧.
Practise this grammar point on paper: 19 exercises using only HSK 2 words, with an answer key on a separate page. Free to print and copy for your class.
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