watashi wa kuroi boorupen de techou ni yotei o kakimasu.

I write my plans in my planner with a black ballpoint pen.

Breakdown of watashi wa kuroi boorupen de techou ni yotei o kakimasu.

• は: topic particle
• 私わたし: I
• を: direct object particle
• で: means particle

Questions & Answers about watashi wa kuroi boorupen de techou ni yotei o kakimasu.

Why is は used after 私 instead of が?

は marks 私 as the topic of the sentence: as for me, ....

In this sentence, 私は sets up who is doing the action, and the rest of the sentence explains what that person does.

If you used が instead, it would sound more like you are specifically identifying me as the person who writes it, often with more emphasis or contrast.

So here:

• 私は = As for me, ... / I ...
• 私が = I am the one who ... (more focus on I)

For a simple neutral sentence like this, は is very natural.

Why is 黒い used before ボールペン?

黒い is an i-adjective, and it directly modifies the noun ボールペン.

So:

• 黒い = black
• ボールペン = ballpoint pen
• 黒いボールペン = a black ballpoint pen

This works just like English adjective + noun order.

A learner may expect 黒のボールペン, and that is also possible in some contexts, but 黒いボールペン is the most straightforward and natural way to say a black ballpoint pen.

Why is the particle で used after ボールペン?