1) Delite "に" after "三時間" from each sentence, and done! You get two correct sentences. 今日、私は三時間日本語を勉強しました。 今日は私は三時間日本語を勉強しました。 2) The two sentences above are almost same in meaning, like the following English sentences are almost the same as well: Today, I studied Japanese for three hours. I studied Japanese for three hours today. (Please do not ask me which one to which ^^;) *If you keep some rules, word order in Japanese is fairly flexible. If you have more interest, please check the web site below. Article #3. 'A language with flexible word order' has this point, and I'd recommend you to read other articles, if you have time :): http://www.nkc.u-tokyo.ac.jp/study_info/study_info01_01_e.html If you have further questions, please feel free to ask me.

I'm not a native speaker. I think the first would be better. but, に is not necessary. 今日、私は三時間日本語を勉強しました。