ですが vs が， what are the differences between them?


In my textbook, there is a sentence:

中国語の発音は難しかったが、漢字はあまり難しくなかった。

And there is also a sentence:

「日本史」は難しくなかったですが、「翻訳」は大変でした。

I'm confusing that is it still a correct expression with removing of the "です"
in "ですが" in the second sentence?

If it is still a native expression, what's the differences between it and the
original one?




難しい = (it) is difficult -- informal  
難しいです = (it) is difficult -- polite  
難しかった = (it) was difficult -- informal  
難しかったです = (it) was difficult -- polite  
難しくない = (it) is not difficult -- informal  
難しくないです = (it) is not difficult -- polite  
etc.  
You get the idea.  
For i-adjective conjugations putting です at the end changes from informal to
polite style.

I think 「日本史」は難しくなかったが、「翻訳」は大変でした without the です would be awkward because the
parts either side of が are independent statements, and the latter ends in でした
which is in polite style. But note that this doesn't apply to all
conjunctions.




難しい = (it) is difficult -- informal  
難しいです = (it) is difficult -- polite  
難しかった = (it) was difficult -- informal  
難しかったです = (it) was difficult -- polite  
難しくない = (it) is not difficult -- informal  
難しくないです = (it) is not difficult -- polite  
etc.  
You get the idea.  
For i-adjective conjugations putting です at the end changes from informal to
polite style.

I think 「日本史」は難しくなかったが、「翻訳」は大変でした without the です would be awkward because the
parts either side of が are independent statements, and the latter ends in でした
which is in polite style. But note that this doesn't apply to all
conjunctions.

