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I am a beginner in Chinese (Mandarin), and I am following a phone app to learn.
However I cannot understand what is the difference between 正在 and 在 ? For example:
你正在干什么？
and
你在干什么？
The app says that they are interchangeable... is it always the case?
For what you are asking 在 and 正在 are basically interchangeable.
You will rarely hear 正在 in colloquial, normal, conversation - it is quite formal and elaborate. For instance looking up 你正在干什么？ gives a result from Google Books reading:
此时你正在干什么？
No one would really talk like this unless you're playing a role in an old-timey kind of show or something like that.
在 you could contest is an abbreviation of 正在 in your given scenario and is the most down-to-earth way to express something currently being underway.
From what I learned, 在 means you're doing something, while 正在 means you're doing something right now. For instance:
我在学习中文。 I'm studying Chinese language (this year, this month, not necessarily in this moment).
我正在学习中文。 I'm studying Chinese language right now (the book is open in front of me, please don't disturb).
Not sure if this difference exists all the time, though.
