all these three words have the meaning of "doing sth", but there are also difference: 1 meaning "正" mainly about the time, "在" mainly about the status, and "正在" covers both time and status, like ---在干吗？---我在做作业呢。 ---what are you doing? ---i'm doing homework. ---在干吗？能来一下吗？ ----我正做作业呢！ ---what are you doing? can you come now? ---i'm doing my homework! here, "正" emphsis on "i have sth to do at the moment now", of course u can use "在", but not so good as "正" here 2 grammar when you use “正", you can not merely use "正+verb"， while it is ok in "在" and ”正在“ ---你在做什么？ ---我在做作业。 ok ---我正在做作业。 ok ---我正做作业。 no----我正做作业呢！ Ok --- you need to use "正+verb+呢/着" 3 在can not be followed by "从"（from）， while 正 and 正在 is ok 他正从学校赶回来。OK 他正在从学校赶回来。OK 他在从学校赶回来。NO 4 在can be used to mean the actions are done again and again or lasts quite long, while 正 and 正在 can not. 正在做作业. doing homework now OK 一直正在做作业 NO ---一直在做作业。 ok you can see when 正在and 在 can only be used in "doing sth now", but not "doing sth all the time"

Is actually the same meaning, expressed carries on when behavior movement and so on ,may equate them.

hello Davrous 1. "在" and "正在" generally are used for forming a sentence with the continuous tense,and they can be interchangeable when they mean "in process of;in course of". The basic sentence structures: （1）某人（sb.）+ 在/正在 + 做某事（doing sth.） （2）某人（sb.）+ 在/正在 + 某地（sp.）+ 做某事（doing sth.） I am/was watching TV at home. 我正在/在家里看电视。 When i was leaving he came back. 我正/正在离开时，他回来了。 2. "正在" sometimes actually are two words "正" and "在".When useing "正在" before "someplace" , "正" is an adverb, it just shows one continuous action or situation. "在" is one preposition, like "in" "at" "on"etc. （1）i'm on the way home. 我正在/在回家的路上。 （2）She's sitting on the couch. 她正坐在沙发上。 （3）He's at school. 他正在/在学校。 "正" sometimes means "just; exactly; right; precisely" : right here and now. 正是此时此地。

正and正在 has the same meaning in Chinese.For example, 我正在看电视。/我正在看电视。（They also means "I am watchting,"）

The sentence "I am writing" can be expressed both as 我在写字 [Wǒ zài xiězì] and 我正在写字[Wǒ zhèngzài xiězì]. The word 正 stresses that action is currently in progression. In sentences such as "我正 想/讲 到你呢, 你就来了 [Wǒ zhèng (xiǎng/jiǎng) dào nǐ ne, nǐ jiù lái le]," people normally would not use "在". However the word "在" is used in the following sentence: "我正在 想/讲 你呢, 你就来了 [Wǒ zhèngzài xiǎng/jiǎng nǐ ne, nǐ jiù lái le]." Hope that's clear.