suitou no huta wo simenai to, kaban no naka ga nuremasu.

Here, 閉めないと means if you don’t close (it).

It comes from:

• 閉める = to close
• 閉めない = not close
• 閉めないと = if you don’t close it, then ...

So in this sentence, 閉めないと、かばんの中がぬれます means:

• If you don’t close the lid, the inside of the bag gets wet.

A useful note: 〜ないと can also sometimes mean have to in everyday speech, but not in this sentence. Here it is clearly a conditional: if you don’t ..., then ...

The particle と here makes a conditional: when/if X happens, Y happens.

In this sentence, it connects:

• 水筒のフタを閉めない = you don’t close the water bottle lid
• かばんの中がぬれます = the inside of the bag gets wet

So the structure is:

• A と、B
• If/when A happens, B happens

This と is especially common when the result is seen as:

• a natural consequence
• a predictable result
• something automatic

That fits this sentence well: if the lid is not closed, the bag gets wet.

Because と often sounds best when the result is an automatic or natural consequence.

In this sentence, the speaker is not talking about a personal decision or a one-time surprise. They are saying something like:

• If you don’t close the lid, your bag will get wet.
• This is just what happens.

That makes と very natural.

Very roughly:

• 〜と = if/when X happens, Y naturally follows
• 〜たら = if/when X happens, then Y; more flexible and common in conversation
• 〜なら = if it is the case that X / if you mean X

So 閉めないと、かばんの中がぬれます sounds like a general warning or fact.

Because 閉める is a transitive verb, and it takes a direct object marked by を.

So:

• フタ = lid
• フタを閉める = to close the lid

The lid is the thing being closed, so it gets を.

Compare:

• ドアを閉める = close the door
• 目を閉じる = close your eyes
• フタを閉める = close the lid

This is a very common question.

• 閉める = transitive = to close something
• 閉まる = intransitive = something closes / is closed

Examples:

• ドアを閉める = I close the door
• ドアが閉まる = The door closes

In your sentence, someone is closing the lid, so Japanese uses the transitive verb:

• フタを閉めない = not close the lid

If you used 閉まる, the meaning would shift toward the lid itself being closed:

• フタが閉まらない = the lid won’t close

The particle の connects nouns. It often works like of, or like an English noun modifying another noun.

So:

• 水筒のフタ = the water bottle’s lid / the lid of the water bottle
• かばんの中 = the inside of the bag

You can think of の here as showing a relationship between two nouns:

• A の B = B related to A

Some common examples:

• 学校の先生 = the school’s teacher / teacher at the school
• 日本の車 = Japanese car / car from Japan
• 箱の中 = inside of the box

Because かばんの中 is the thing that gets wet, so it is the subject of ぬれます.

Break it down:

• かばんの中 = the inside of the bag
• が = marks the subject
• ぬれます = gets wet

So the sentence is saying:

• The inside of the bag gets wet.

This is why が is natural here.

If you said:

• かばんがぬれます

that would mean:

• The bag gets wet

That is also possible, but it is slightly different. The original sentence focuses specifically on the inside.

Because ぬれる means to get wet, while ぬらす means to make something wet.

• ぬれる = intransitive
• ぬらす = transitive

In this sentence, the inside of the bag is not actively wetting something else. It itself becomes wet. So Japanese uses:

• かばんの中がぬれます = the inside of the bag gets wet

Compare:

• 雨で服がぬれた = My clothes got wet because of rain
• 床をぬらした = I made the floor wet

Yes. フタ and 蓋 mean the same thing: lid or cap.

It is very common to write this word in katakana as フタ, even though there is a kanji, 蓋.

Why?

• The kanji 蓋 is not very common for many learners
• Katakana is often used for readability
• Some everyday items are frequently written in katakana even when they are not foreign words

So 水筒のフタ is completely normal.

Yes, in other contexts it can.

In casual Japanese, 〜ないと is often used as a shortened form of:

• 〜ないといけない
• 〜なければならない

Both mean have to.

For example:

• もう行かないと。
• I have to go now.

But in your sentence, that is not what it means, because the sentence continues with a result:

• 閉めないと、かばんの中がぬれます
• If you don’t close it, the inside of the bag gets wet

So here it is definitely the conditional if you don’t ...

It can feel like a warning or a statement of consequence.

The sentence is not a direct command like:

• フタを閉めてください = Please close the lid

Instead, it says what will happen if you do not do it:

• If you don’t close the lid, the inside of the bag gets wet.

That makes it sound like:

• a warning
• a practical reminder
• a general fact about what happens

This kind of wording is very common in Japanese instructions and everyday advice.

You could say that in natural English, especially if you are simplifying.

But the Japanese specifically says:

• かばんの中 = the inside of the bag

So the most accurate translation keeps that idea:

• the inside of the bag will get wet
• the inside of your bag will get wet

If you just say your bag will get wet, the meaning is still close, but you lose the specific detail that it is the inside that gets wet.