一起是副词（adv.） 在一起是介词（prep.）短语，作状语

In fact, it'll be a very simple problem if you see them in this way: 在一起can be both a verb phrase or a adv. For example: 我们在一起。 在一起here means 'ARE together'. Or you can say 我们在一起吃饭。在一起here means 'together', as a adv. While,一起is only just a adv. For example:you can say我们一起吃饭. If you say 我们一起, people would understand you, which means 'Let's.' but that's not a complete sentence, it just happens in very informal and casual situations. However, I don't believe a well-educated person would say something like '我们一起'. You can both say 我们一起吃饭 and 我们在一起吃饭, when they are both ADV, while what's the difference? 在shows it is happening, it's like the present continuous tense in English grammar. In Chinese most words with '在' means 'now', like: 现在、正在 etc. But if you say 我们一起吃饭，without "在", it could be a simple present or a future tense, depends on the context. To sum up, 一起and在一起 can both used as adv, but 在一起can also used as a verb phrase. When they are both advs, 在emphasises that the thing is happening.

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Hello Cathy, They both mean "together" with some differences in meaning depending on the context ;they could also mean " in the same place, with ,altogether ( in total). 一 起 (yīqĭ ) as an adverb it means "with each other " 我 们 一 起 骑 了 很 长 一 段 时 间 的 自 行 车 。 We went on long bicycle rides together. 在 一 起 ( zài yīqĭ ) Could mean: - in the same place 这 些 树 长 拢 在 一 起 The trees grew close together. - in a group 把 这 些 原 料 充 分 地 合 在 一 起 。 Mix the ingredients together thoroughly.

在一起=be together, is a verb as you can see. while 一起=together