japanese grammer?

Question

when do we use そですand when we use でそです。。。。。ex: 雨がふるそです　
　　　　　　ex:曇りでそです

Answer 1

With そう　you really have to pay attention to the form of the preceding predicate because depending on that, it determines whether or not what you're saying is:

1) They say X (hearsay)
2) looks like, seems like X.

In addition to that, different word classes, nominals, adjectivals, and verbs have different rules for how they are used with そうです。

In your example sentences it is used in a way that someone is reporting something.

雨がふるそうです。They say it will rain. (I hear it's going to rain.)

曇りだそうです。　They say it's cloudy. (I hear it's cloudy.)

In the first sentence, preceding そう is a verb. Therefore, correct usage in this case is the direct form of the verb plus そうです。

In the second sentence, preceding そう is a nominal.( noun) So in this case, you have to use だ　before そう.

I hope this is clear.

Answer 2

When you use そうです to express hearsay, just attach そうです to the basic sentence.
雨がふる it will rain
->雨がふるそうです。They say it will rain.　
曇りだ it is cloudy weather
->曇りだそうです。They say it is cloudy (weather).

When you use そう(です) to guess at an outcome, it's very complicated syntactically and semantically.
I hope this site will help you.
http://www.guidetojapanese.org/similar.html