对还是错?

中文不很简单。如果我学习很多，我就累(我累了？)。因为我很累，所以早一点儿睡觉。

中文有点儿难。如果我学习久了，我就会感觉很累。因为很累，所以我想早一点休息。

In Chinese we will not often to say Chinese language is not simple, we always say Chinese language is a little bit hard. 久（jiu） meaning time long, in your stencense I think perhaps 久 is better. And the second part I think you want to express because I am tired so I want to go to bed early. 休息（to have a rest） more Chinese. Aber hast du gut gemacht. :-)

对还是错?

中文不是很简单。如果我学了很久，我就会累了。因为我很累了，所以早一点儿睡觉。

对还是错?

中文不是很简单。如果我学习很久，我就会累。因为我很累，所以想（要）早一点儿睡觉。

对还是错?

中文不很简单。如果我学太久习很多，我就会累了(我累了？)。因为我很累，所以因此早一点去儿睡觉。

There are many ways to correct this because of the nuances can be vastly different.  It's best when learning a language, to provide your native language version so that we can help your intended nuances expressed more precisely.  However, I don't know any German :(

对还是错?

中文不好学。我学习久了，就累了(我累了？)，所以想早一点儿睡觉。