在 / 再 how to know what is it??
Some classes ago I learned that 在 means 'to be' and 在 means 'again'. And I also learned that if I say 我在说 it means 'I'm speaking' and if I say 我再说 means 'I speak again'.

If I read it I will understand perfectly, but I only HEAR what would be writen with pinyin 'wo3 zai4 shuo1', how to distinguish if it's 在 or 再 ???

Question

在 / 再 how to know what is it??
Some classes ago I learned that 在 means 'to be' and 在 means 'again'. And I also learned that if I say 我在说 it means 'I'm speaking' and if I say 我再说 means 'I speak again'. 

If I read it I will understand perfectly, but I only HEAR what would be writen with pinyin 'wo3 zai4 shuo1', how to distinguish if it's 在 or 再 ???

Answer

Gernerally, we don't speak "我在说" or "我再说" all alone...there will be a sentence. 
To be precise, you should understand it with a context. 
For example, independently , if we say "我在说他的事情" , or "我再说他的事情" , we can't distinguish it.
Look at the conversations  "你在干嘛?"
                                       "我在说他的事情."  
In this situation, we don't say "我再说他的事情."

You comprehend?                   

And for conventional reason, the chinese won't say "我再说他的事情" , we will say "我再说一下(一次)(一遍)他的事情"

Answer

“在” 1.表示 "正在" （present progressive）  eg：我(正）在看书 i am reading.
2.介词preposition，你在哪？where r u ?我在学校。i am in school.
在……中，在……上，在……下，eg:在学习中，(表示的意思比较抽象abstract)
“再”表示 once again   eg:wo zai shuo yi ci.我再说一次。i speak again.

Answer

"在"means "at" or "doing sth" but "再" means "again"=“又”
example:
他在哪儿？-Where IS he?  你在玩么？-ARE you playing?
他再一次输了=他又输了-He fails again.

Answer

Don't worried about that .Senteces or words are always used in a certain context in real life