日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Here's the other lang-8 correction I got:
私は真面目日本語の学生だ。 becomes
私は真面目に日本語を学ぶ学生だ。

I don't doubt the correction, can someone just explain it to me?