kono senzai o tsukaeba, shatsu no shimi ga ochiru to omoimasu.

使えば is the -ba conditional form of 使う (to use).

So:

• 使う = to use
• 使えば = if you use it / when you use it

In this sentence, この洗剤を使えば means if you use this detergent.

This -eba form often gives a fairly neutral if meaning: if this condition is met, then the following result is expected.

Because 洗剤 is the direct object of 使う.

• 洗剤を使う = to use detergent

The particle を marks the thing being used. So:

• この洗剤を使えば = if you use this detergent

Because の is linking shirt and stain in a possessive/descriptive way.

• シャツのしみ = the shirt's stain / the stain on the shirt

Here, の shows that the stain belongs to or is associated with the shirt.

So this phrase is not saying shirt is an object of a verb. It is simply naming the stain on the shirt.

Because しみ is the subject of 落ちる.

• しみが落ちる = the stain comes off / the stain comes out

This is a very common pattern with intransitive verbs in Japanese:

• the thing that undergoes the change takes が

Here, the stain is what comes off, so it is marked by が.

This is an important intransitive/transitive pair:

• 落ちる = to fall off / come off / come out (intransitive)
• 落とす = to remove / drop / make something come off (transitive)

In this sentence:

• しみが落ちる = the stain comes out

The focus is on the stain disappearing or coming off, not directly on a person actively removing it.

If you said しみを落とす, that would mean to remove the stain.

So both ideas are related, but this sentence uses the intransitive style:

• the stain comes out rather than
• someone removes the stain

Here, と marks the content of a thought.

• 落ちると思います = I think that it will come out

You can think of と as functioning like that in English:

• [シャツのしみが落ちる] と 思います
• I think [the shirt stain will come out]

Japanese often uses と before verbs like:

• 思う = think
• 言う = say
• 知っている = know

Using と思います makes the statement softer and less absolute.

Compare:

• シャツのしみが落ちます。 = The shirt stain will come out.
• シャツのしみが落ちると思います。 = I think the shirt stain will come out.

The version with と思います sounds more like an opinion, expectation, or polite judgment. It is very natural when someone is recommending something but does not want to sound too forceful or 100% certain.

Because before と思います, Japanese normally uses the plain form.

So:

• 落ちる
  • と思います = correct
• 落ちますと思います = unnatural / not standard

This is a very common rule: the clause before と is usually in plain form.

Examples:

• 行くと思います = I think he/she will go
• 難しいと思います = I think it is difficult
• 大丈夫だと思います = I think it is okay

この means this and directly modifies the noun that follows it.

• この洗剤 = this detergent

Japanese has this pattern:

• この + noun = this ...
• その + noun = that ...
• あの + noun = that ... over there

So この must be followed by a noun. You would not say この by itself here.

Not necessarily by itself.

洗剤 is a general word for detergent / cleaning agent / cleanser. The exact meaning depends on context.

In this sentence, because it talks about a shirt stain, many learners would naturally understand it as something like:

• detergent
• stain remover
• laundry detergent

But the word itself is broader than just one specific product type.

Yes, 使うと is possible in many contexts, but the nuance changes a bit.

• この洗剤を使えば、シャツのしみが落ちると思います。 = If you use this detergent, I think the shirt stain will come out.

• この洗剤を使うと、シャツのしみが落ちると思います。 = When/if you use this detergent, I think the shirt stain will come out.

Very roughly:

• -ば often sounds like a straightforward condition: if
• -と often suggests a more automatic or natural result: when/if X happens, Y happens

In a sentence like this, 使えば sounds very natural because it presents using the detergent as the condition for the expected result.

Because the sentence is focusing more on the grammatical relationships than on setting a contrastive topic.

The key structure is:

• この洗剤を使えば = if you use this detergent
• シャツのしみが落ちる = the shirt stain comes out
• と思います = I think

Here:

• を marks the object of 使う
• が marks the subject of 落ちる

If you changed some particles to は, the nuance would shift. For example:

• シャツのしみは落ちると思います can sound more like as for the shirt stain, I think it will come out

That is possible in some contexts, but the original sentence is a more neutral presentation of the result.

Formally, 落ちる is in the Japanese non-past form.

Japanese non-past can cover:

• present
• future
• general truths/habitual results

Here it is best understood as a predicted result:

• the stain will come out or
• the stain comes out

Because the sentence is conditional, English usually uses future-style wording:

• If you use this detergent, I think the stain will come out.

That is normal in Japanese.

In this sentence:

• この is usually written in hiragana
• 洗剤 and 思います and 落ちる are commonly written with kanji
• しみ is often written in hiragana, even though kanji forms exist

Japanese writing mixes:

• kanji for many content words
• hiragana for grammar and many common native words
• sometimes katakana for emphasis, foreign words, or certain product-related terms

So the mix here is very natural. Also, in learner materials, some words may be written in hiragana even if a kanji exists, just to make reading easier.

It breaks down like this:

• この洗剤を使えば
  = if you use this detergent

• シャツのしみが落ちる
  = the stain on the shirt will come out

• と思います
  = I think

So the full structure is:

[If you use this detergent], [the shirt stain will come out], [I think].

A very common Japanese pattern is:

• condition + result + と思います

This lets the speaker give an opinion about what will happen under some condition.