atarashii senzai o tsukattara, shatsu no shimi ga yoku ochita.

When I used a new detergent, the stains on the shirt came out well.

Breakdown of atarashii senzai o tsukattara, shatsu no shimi ga yoku ochita.

• を: direct object particle
• が: subject particle
• の: possessive case particle
• 新あたらしい: new

Questions & Answers about atarashii senzai o tsukattara, shatsu no shimi ga yoku ochita.

What does 使ったら mean here?

使ったら is the たら-form of 使う (to use).

In this sentence, it means something like:

• when I used
• after I used
• if I use / if someone uses (depending on context)

Here, the most natural reading is when/after I used the new detergent.

So:

• 新しい洗剤を使ったら = when I used the new detergent / after using the new detergent

The たら form often connects one event to the result that followed. In this sentence, the result is that the shirt stains came out well.

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Why is を used after 洗剤?

Because 洗剤 is the direct object of 使う (to use).

• 洗剤を使う = to use detergent

The particle を marks the thing that is used.

So:

• 新しい洗剤を使ったら = when I used the new detergent

This is very standard:

• ペンを使う = use a pen
• 日本語を使う = use Japanese
• アプリを使う = use an app

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Why is it シャツのしみ? What does の mean here?