tyuui wo sinai to, saihu wo ototosite simaimasu.

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Questions & Answers about tyuui wo sinai to, saihu wo ototosite simaimasu.

In the phrase 注意をしないと, what does the construction ~しないと mean?
The pattern ~しないと is a casual conditional meaning “if you don’t (do ~)”. Here, 注意をしないと means “if you don’t pay attention.”
Why is 注意 followed by and しない? Isn’t 注意 a noun?

Yes, 注意 is a noun meaning “attention” or “caution.” To turn it into a verb phrase, Japanese uses をする:
注意をする = “to pay attention”
In the negative, 注意をしない = “not pay attention.”

What role does the particle play after 注意をしない?
The particle marks a conditional meaning “if/when.” So 注意をしないと = “if you don’t pay attention.”
Why is the verb in the second clause 落としてしまいます instead of simply 落とします?

The auxiliary ~てしまう adds nuance of regret or unintended result.
落とします = “drop” (neutral)
落としてしまいます = “end up dropping” or “drop regrettably”

It shows the speaker sees dropping the wallet as an unfortunate accident.

Why is the progressive/te-form 落として used before しまいます?

To attach the auxiliary しまう, you need the te-form of the main verb.
落とす → te-form 落として → plus しまいます yields 落としてしまいます.

Could this sentence be made more casual? How?

Yes.
• Drop the polite ~ます: 落としちゃう instead of 落としてしまいます.
• You might also omit some particles: 注意しないと財布落としちゃうよ。
This sounds more conversational and less formal.