bentoubako wo aratta ato, akeppanasi ni site yoku kawakasimasu.

を marks the direct object of the verb 洗った.

So in 弁当箱を洗った, the lunch box is the thing being washed.

• 弁当箱 = lunch box / bento box
• を = object marker
• 洗った = washed

Even though the second part of the sentence is about drying it, the object is stated only once at the beginning and understood for the rest of the sentence.

In Japanese, when あと means after doing..., the verb before it is usually in the plain past form.

So:

• 洗ったあと = after washing
• literally, the after of having washed

This does not mean the whole sentence is in the past. It just shows that the washing happens first, and then the next action happens after that.

Compare:

• 食べたあと = after eating
• 帰ったあと = after going home

This is a very common pattern.

Here, あと means after.

The structure is:

• Verb (past plain) + あと = after doing X

So:

• 洗ったあと、開けっぱなしにしてよく乾かします。 = After washing it, I leave it open and dry it well.

It connects the first action to the next actions in sequence.

開けっぱなし means left open or left in the open state, often with the nuance that something stays that way continuously.

It comes from:

• 開ける = to open
• 〜っぱなし = left as is; left in that state

So 開けっぱなし means something like:

• open and left that way
• left open

In this sentence, it means the lunch box is kept open so it can dry properly.

Often, yes. 〜っぱなし frequently has a nuance like:

• left on
• left open
• left as is
• not taken care of afterward

For example:

• 電気をつけっぱなしにする = leave the lights on
• ドアを開けっぱなしにする = leave the door open

Those can sound careless depending on context.

But in this sentence, it is not really negative. Leaving the lunch box open is intentional and useful, because it helps it dry. So the nuance is more practical than careless.

The に marks the resulting state.

The pattern is:

• [state / condition] + にする = make something into that state / leave it in that state

So:

• 開けっぱなしにする = leave it open / keep it in an open state

Here it appears in the て-form as にして, because it connects to the next verb:

• 開けっぱなしにして、よく乾かします = leave it open and dry it well

Yes. It is the て-form of する.

• する → して

But here する is not just the general do. In this pattern, it means to make/keep something in a certain state.

So:

• 開けっぱなしにする = leave it open
• きれいにする = make it clean
• 静かにする = make it quiet / be quiet

In your sentence, 開けっぱなしにして links the state left open to the next action dry it well.

Here, よく means well or thoroughly, not often.

よく can mean different things depending on context:

• よく行きます = I go often
• よく見てください = please look carefully
• よく乾かします = dry it well / thoroughly dry it

In this sentence, よく乾かします means making sure the lunch box dries completely and properly.

Because 乾かす and 乾く are different verbs:

• 乾く = to dry, to become dry (intransitive)
• 乾かす = to dry something, to make something dry (transitive)

In this sentence, the speaker is actively drying the lunch box, so 乾かします is used.

Compare:

• 弁当箱が乾く = the lunch box dries
• 弁当箱を乾かす = dry the lunch box

Even though を does not appear again before 乾かします, the object 弁当箱 is still understood.

Because Japanese often omits words that are already clear from context.

The full idea is something like:

• 弁当箱を洗ったあと、弁当箱を開けっぱなしにして、弁当箱をよく乾かします。

But repeating 弁当箱 that many times would sound unnatural. Once the topic/object is clear, Japanese usually leaves it out.

So the sentence naturally keeps only one mention of 弁当箱 and lets the rest be understood.

The subject is omitted, which is very normal in Japanese.

Depending on context, it could mean:

• I wash it, leave it open, and dry it well.
• You should wash it, leave it open, and dry it well.
• One washes it, leaves it open, and dries it well.

Without context, Japanese often does not specify the subject if it is obvious from the situation.

In a plain explanatory sentence like this, English often supplies I or a general you.

Yes, and the meaning would be very similar.

• 開けっぱなしにして = leave it open
• 開けたままにして = leave it open / keep it open

The nuance is a little different:

• 〜っぱなし can suggest something is left that way continuously, sometimes carelessly
• 〜たまま is more neutral and simply describes remaining in that state

So in this sentence:

• 開けっぱなしにして sounds natural and common
• 開けたままにして would also be correct, and may sound a bit more neutral

Yes, it is polite because it ends with 乾かします, which is in the ます-form.

But it is a mix of styles that is completely normal in Japanese:

• 洗った = plain past
• 乾かします = polite

This happens often when a non-final clause is followed by a polite final clause. The politeness is usually shown mainly at the end of the sentence.

A fully casual version might be:

• 弁当箱を洗ったあと、開けっぱなしにしてよく乾かす。

A polite version like your sentence is natural in explanations, instructions, or daily conversation.

The sentence breaks down like this:

• 弁当箱を = the lunch box
• 洗ったあと、 = after washing it,
• 開けっぱなしにして = leaving it open,
• よく乾かします。 = dry it well

So the flow is:

1. wash the lunch box
2. leave it open
3. dry it thoroughly

It is a sequence of connected actions, which is a very common Japanese sentence pattern.