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

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.

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

Here, の connects two nouns and shows a relationship between them.

• シャツ = shirt
• しみ = stain
• シャツのしみ = the shirt’s stain(s) / stains on the shirt

In English, we would usually say stains on the shirt rather than the shirt’s stains, but Japanese often uses A の B for this kind of relationship.

Other similar examples:

• 服の汚れ = dirt on clothes
• 机の上 = on the desk / the top of the desk
• 紙の色 = the color of the paper

So シャツのしみ simply means the stains belonging to or found on the shirt.

This is one of the most important points in the sentence.

The verb here is 落ちる, which is an intransitive verb. It means:

• to fall
• to come off
• to come out (for stains, dirt, makeup, etc.)

Because 落ちる is intransitive, the thing that comes off is marked with が, not を.

So:

• しみが落ちた = the stains came out
• literally: the stains fell off / came off

Compare:

• しみが落ちる = the stain comes out
• しみを落とす = remove the stain

Here is the pair:

• 落ちる = something comes off by itself / as a result
• 落とす = someone removes something

That is why the sentence uses しみが落ちた, not しみを落ちた.

Japanese often describes the result from the stain’s point of view rather than from the person’s point of view.

So instead of saying:

• I removed the stains well

Japanese naturally says:

• the stains came out well

That is:

• しみがよく落ちた

This is very common with intransitive verbs in Japanese. The language often focuses on what happened, not necessarily who caused it.

If you wanted to say I removed the stains, you could use the transitive verb 落とす:

• 新しい洗剤を使ったら、シャツのしみをよく落とせた。
• When I used the new detergent, I was able to remove the shirt stains well.

But the original sentence sounds very natural and emphasizes the result.

Not in this sentence.

よく can mean different things depending on context:

1. well
2. nicely
3. a lot / thoroughly
4. often

Here, with 落ちた, it means well or thoroughly.

So:

• しみがよく落ちた = the stains came out well
• or the stains came out easily / thoroughly

It does not mean often here.

Compare:

• 日本語をよく勉強する。 = I often study Japanese.
• この洗剤は汚れがよく落ちる。 = This detergent removes dirt well.

Same word, different meaning based on context.

落ちた is the plain past form of 落ちる.

• 落ちる = come off / come out
• 落ちた = came off / came out

The sentence describes a past experience:

• I used the new detergent, and the stains came out well.

So the past tense is natural.

Also, with たら, the first part is also in the past form:

• 使ったら

But in grammar terms, たら is not simply plain past. It is a conditional/sequence form built from the past form. In this sentence, it gives the sense of:

• when I used it
• after I used it

Yes, the subject is omitted, which is very normal in Japanese.

The sentence does not explicitly say I, we, or someone. But from context, a listener would usually understand something like:

• I used the new detergent
• and the stains on the shirt came out well

Japanese often leaves out subjects when they are obvious from context.

So even though there is no written 私が, the sentence naturally suggests the speaker is talking about their own experience.

A more explicit version would be:

• 私が新しい洗剤を使ったら、シャツのしみがよく落ちた。

But in everyday Japanese, that would usually be unnecessary.

Because 新しい is an i-adjective modifying the noun 洗剤.

In Japanese, adjectives usually come before the noun they describe:

• 新しい洗剤 = new detergent
• 白いシャツ = white shirt
• 古い本 = old book

So:

• 新しい 洗剤 = new detergent

This works much like English word order in this case.

It could be replaced in some situations, but the nuance would change.

1. 使ったら

This is the original form, and it works very naturally for:

• a real experience in the past
• a result discovered after doing something

So here it feels like:

• When I used the new detergent, the stains came out well.

2. 使うと

This often sounds more like:

• a general result
• something that happens whenever you do it

Example:

• この新しい洗剤を使うと、シャツのしみがよく落ちる。
• If you use this new detergent, shirt stains come out well.

This sounds more like a general fact or product claim.

3. 使えば

This means if you use and often focuses on condition rather than a discovered result.

Example:

• 新しい洗剤を使えば、シャツのしみがよく落ちる。
• If you use the new detergent, shirt stains will come out well.

So the original 使ったら is best if the speaker is talking about what happened in one actual case.

They are related, but not exactly the same.

しみ

Usually means a stain or spot, especially one that has soaked into fabric or a surface.

Examples:

• coffee stain
• sweat stain
• food stain

汚れ

Means dirt, filth, stain, or smudge more broadly.

So:

• しみ is more specific
• 汚れ is more general

In this sentence, しみ is a good choice because we are talking about stains on a shirt.

Also, しみ is often written in kana, though you may also see シミ in some contexts.

Yes, it is very common.

The basic meaning of 落ちる is to fall, but Japanese uses it in wider ways than English does. For stains, makeup, dirt, paint, and similar things, 落ちる often means:

• come off
• come out
• be removed

So:

• しみが落ちる = a stain comes out
• メイクが落ちる = makeup comes off
• 汚れが落ちる = dirt comes off

Even if the literal image is falling off, the natural English translation is usually come out or come off.

So yes, it is completely normal Japanese.

Yes, very easily.

Because it says:

• I used the new detergent
• the stains on the shirt came out well

it sounds like the speaker is reporting a successful result. That kind of sentence could appear in:

• casual conversation
• a product review
• a recommendation
• an advertisement-like comment

If spoken with the right tone, it could imply:

• This detergent works well
• I’m impressed with this detergent

But grammatically, the sentence itself simply reports what happened. Any stronger feeling, such as surprise or approval, comes from context and tone.