watashi wa nikki ni eiga no kansou o kakimasu.

topic particle

Breakdown of watashi wa nikki ni eiga no kansou o kakimasu.

• 私わたし: I
• を: direct object particle
• 映えい画が: movie

Questions & Answers about watashi wa nikki ni eiga no kansou o kakimasu.

Why does the sentence use は after 私 instead of が?

は marks the topic of the sentence: what we’re talking about.
So 私は means roughly “As for me,” or “Speaking about me,”.
が marks the grammatical subject, especially when introducing new information or contrasting who actually did something.
Here, the speaker is just setting themselves up as the topic (we’re talking about what I do), so 私は is natural. Using 私が would sound more like you’re contrasting with someone else: “I (not someone else) will write my impressions in my diary.”

Can I drop 私 and just say 日記に映画の感想を書きます?

Yes. In natural Japanese, the subject (私) is often omitted when it’s clear from context.
日記に映画の感想を書きます will normally be understood as “I write / will write my impressions of the movie in (my) diary.”
You keep 私は if you particularly want to emphasize “me” or contrast yourself with others, or if it’s the first time you’re clearly establishing who the subject is.

What exactly does 日記に mean? Why use に here, not で?