imouto ha hon wo tukue no ue ni okippanasi ni surukoto ga ooi.

置きっぱなし means left as is after putting something down, often with the nuance that the person didn’t bother to put it away.

It comes from:

• 置く = to place, put
• 〜っぱなし = leaving something in that state; doing something and not following through properly

So 置きっぱなし is not just placed. It implies left there carelessly / left lying around.

In this sentence, it describes books being left on the desk instead of being cleaned up or returned to where they belong.

In 置きっぱなしにする, the にする pattern means to make something into that state or to leave something in that state.

So:

• 置きっぱなし = the state of having been left sitting there
• 〜にする = to make/leave it that way

Together:

• 本を置きっぱなしにする = to leave a book lying around / to leave a book where it was placed

This is similar to other patterns like:

• 部屋をきれいにする = make the room clean
• そのままにする = leave it as it is

することが多い means often does or there are many occasions when someone does something.

The structure is:

• dictionary form of a verb phrase + ことが多い

So here:

• 置きっぱなしにすることが多い = often leaves things lying around

This pattern is very common for describing habits or frequent behavior:

• 遅刻することが多い = often comes late
• 朝ごはんを食べないことが多い = often doesn’t eat breakfast

It is a little more descriptive and less direct than simply saying よく〜する, though in many cases the meaning is similar.

Because 本 is the direct object of 置く / 置きっぱなしにする.

The thing being placed and then left there is the book:

• 本を = the book(s)

Meanwhile, the place where it ends up is marked separately:

• 机の上に = on the desk

So the basic relationship is:

• 本を置く = put a book
• 机の上に置く = put it on the desk

Even though the sentence is about the book remaining there, grammatically the book is still the object of the action.

Because 置く is a verb of placement, and に marks the destination or resulting location.

• 机の上に置く = put it onto/on the desk

With で, you usually mark the place where an action happens:

• 机の上で読む = read on the desk
• 教室で勉強する = study in the classroom

But 置く focuses on where something is placed, so に is the normal choice.

は marks 妹 as the topic: as for my younger sister...

The sentence is presenting a general fact or habit about her:

• 妹は、本を机の上に置きっぱなしにすることが多い。
• As for my younger sister, she often leaves books on the desk.

If you used が, it would sound more like identifying her specifically, depending on context:

• 妹が could sound like it is my younger sister who...

For describing someone’s usual behavior, は is very natural.

Japanese often omits things that are clear from context.

So 妹 by itself often means:

• my younger sister when the speaker is talking about their own family
• or simply younger sister if the relationship is already understood

In English, we usually need my younger sister, but Japanese often leaves that unstated unless clarification is needed.

Yes, 〜っぱなし has a somewhat casual, conversational feel.

It is very common in everyday speech and writing, but it can sound less neutral than more formal alternatives.

For example:

• 本を机の上に置いたままにする = leave the book on the desk
• 本を机の上に放置する = leave the book unattended / neglectfully leave it there

Compared with those:

• 置きっぱなし feels natural and everyday
• it often carries a mild feeling of criticism, like just leaving things lying around

So this sentence sounds like a normal, natural way to complain lightly about someone’s habit.

They are similar, but the nuance is a little different.

• 置きっぱなしにする
  • strongly suggests leaving it there without taking care of it
  • often sounds careless, lazy, or forgetful
• 置いたままにする
  • more neutrally means leave it as it was after placing it there
  • does not always sound careless

So:

• 本を机の上に置きっぱなしにする
  • often leaves books lying around on the desk
• 本を机の上に置いたままにする
  • leaves books on the desk without moving them

The first one sounds more like a bad habit.

It can mean either, depending on context, but here it most naturally means books in general.

Japanese nouns do not usually show singular vs. plural explicitly. So:

• 本 can mean a book
• or books

Because the sentence describes a repeated habit, English often translates it naturally as:

• She often leaves books on the desk

rather than:

• She often leaves a book on the desk

Yes. The key part is ことが多い, which strongly suggests a repeated tendency or habit.

So this sentence is not about one time when she left a book on the desk. It means something like:

• She often does this
• This tends to happen a lot with her

That habitual meaning is one of the most important things in the sentence.