When should I prefer to use only 正, only 在 or both together? Are there any rules? Does it sound really weird or incorrect to native speakers if someone use them improperly?
2Can we get some examples of your "usage"?– Mou某 ♦May 4, 2014 at 14:07
正 in itself has many meanings, one of which is "just", and thus can be used in conjunction with 在.
正在 is what you are doing at this very moment, or where you are right now, or in general what is currently present, the progressive aspect (the English -ing form) if you will.
我们正在吃饭 = We are eating
西方正在经历的艰难 = The economic hardship that the West is presently enduring
我正在北京 = I am currently in Beijing (I just got here)
我在北京 = I am in Beijing (more or less stationary)
You could not use 正 alone in a similar fashion.
The progressive 正在 could also be contrasted with the continuous aspect using 着. 我正在穿衣服 = I am getting dressed (current action), while 我穿着衣服 = I am wearing clothes (current state).
Then it gets more tricky when you use both simultaneously, as in 我正在爱着你.
1Actually you can use 正 without 在, such as "我们正喝着酒，他就来了". [Offtopic: 我正在爱着你 sounds kinda weird to me]– RephinxMay 5, 2014 at 7:10
I agree with @Rephinx 我正在爱着你 does sound awkward. The act of love is (usually) so long-lasting that it's considered a habitual state, which you can't use 正在 for. 正在 should be used for actions-in-progress that are expected to finish in the forseeable future. May 6, 2014 at 2:20
It's worth noting that 正在 and 在 do not always refer to the present tense: 昨天我在图书馆的时候，我正在看书。 When I was at the library yesterday, I was reading a book. This makes the "something happening at this moment" definition that is often given quite confusing. A better definition would be "indicates something happening right now (or 'then', if the time context is not the present)" In English, we can say, "When I was at the library yesterday I read a book" or "When I was at the library yesterday, I was reading a book" They have the same meaning. I'm not sure if that is the case in Chinese, though. Nov 25, 2014 at 22:01