kanojo wa mainichi nihongo de nikki o kaite iru kara, nihongo ga dondon jouzu ni naru wake desu.

She writes a diary in Japanese every day, so that's why her Japanese keeps getting better and better.

Breakdown of kanojo wa mainichi nihongo de nikki o kaite iru kara, nihongo ga dondon jouzu ni naru wake desu.

• は: topic particle
• です: to be
• を: direct object particle

Questions & Answers about kanojo wa mainichi nihongo de nikki o kaite iru kara, nihongo ga dondon jouzu ni naru wake desu.

Why is は used after 彼女, but が used after 日本語 later in the sentence?

彼女は marks her as the main topic of the sentence: as for her...

Later, 日本語が marks Japanese ability as the thing that is changing or being described. With 上手になる (to become good/skilled), the thing that becomes good is commonly marked with が.

So the structure is roughly:

• 彼女は = as for her
• 日本語が上手になる = her Japanese becomes good / she becomes good at Japanese

In other words, は sets the topic, while が marks what is specifically doing or receiving the description in that part of the sentence.

Why is で used in 日本語で?

Here, で shows the means or medium used to do something.

So 日本語で日記を書く means to write a diary in Japanese.

Some common uses of で are:

• place where an action happens: 学校で勉強する
• tool/means: ペンで書く
• language/medium: 日本語で話す, 英語で書く

So in this sentence, 日本語で does not mean at Japanese or with Japanese literally. It means using Japanese.