現在形と過去形の違いですが、状況によって使い分けます。その時々によってニュアンスを理解する必要があるでしょう。

I'm learning japanese as well I'm taking a program but it's not an easy language by any means. I thought it would go much faster but I'm getting to conversational level and I can read some Kanji. I found the review for the program on http://bit.ly/1cvbAOg.