There are two particles indicate "in/at" a place in Japanese. And, they are "に"and"で". The difference between of the two is: 1.Place"で" take an action. e.g. 私は日本で英語を教えます。:I teach English in Japan. In this case, you are taking an action of "teach" in Japan. 2.Place "に"existance. e.g.私は日本にいます。：I am in Japan. In this case, you are only mentioning that you are existing in Japan. "に" is also the particle, which indicates moving towards a place as merooka san mentioned earlier. e.g. 私は日本にいきます。：I am going to Japan. 私は日本にかえります。：I am returning to Japan. 　友だちが日本からオーストラリアにきます。：My friend is coming from Japan to Australia. As you can see "に" in this case is indicating the movement from a place to the other.

'で’ is action maker as 'に' is direction maker. You might be able to think in this way. You can use 'で' when you are doing something in a place literally or figuratively. Some examples... 1) The action of reading is taken place on the train. わたしは電車で本を読みます。 I read the book on the train. 2) The action of saying something is taken place in your mind. 心で何か言います。 I say something in my mind. And 3) Teaching English is taken place in Japan. So... わたしは日本で英語を教えます。 is correct. And usage of 'に'. This is the direction maker. Therefore, 'に' indicates that a certain action is doing toward someone or something. Some examples... 1) When the action of moving to Japane is taken place. わたしは日本に行きます。 I go to Japan. 2) When the action of talking to someone is taken place. わたしは彼に話します。 I talk to him. And 3) if you say that 'わたしは日本に英語を教えます'. The meaning goes like this... I teach English language to the country of Japan. It doesn't meke a sense, does it?