syuunou ga tarinai to omou nara, ookii hondana wo kattara dou?

Because 足りない works like an adjective meaning to be insufficient / to not be enough, so the thing that is insufficient is usually marked with が.

• 収納が足りない = storage is insufficient / there isn’t enough storage
• Here, 収納 is the thing being described as insufficient.

Using を would not be natural with 足りない in this sentence.

You may also hear:

• 収納が足りない
• 収納は足りない

The version with が is more neutral and direct. は would give more of a contrastive feeling, like as for storage, it’s not enough.

In this sentence, 収納 means storage space or room to store things, not just the act of storing.

So 収納が足りない means:

• there isn’t enough storage
• you don’t have enough space to put things

This is a very common Japanese use of 収納 in everyday life, especially when talking about homes, furniture, closets, shelves, and organization.

足りない means not enough or insufficient.
It is different from ない, which just means does not exist / there is none.

Compare:

• 本がない = There are no books
• 本棚がない = There is no bookshelf
• 収納が足りない = There is storage, but it is not enough

So 足りない implies that something exists, but the amount is insufficient.

Here, と marks the content of what someone thinks. It works a lot like that in English.

• 収納が足りないと思う = I think that there isn’t enough storage

Structure:

• [plain statement] + と思う
• X と 思う = to think X

So:

• 収納が足りない = there isn’t enough storage
• 収納が足りないと思う = to think there isn’t enough storage

Adding と思う makes the condition more subjective:

• 収納が足りないなら = if there isn’t enough storage
• 収納が足りないと思うなら = if you think there isn’t enough storage

So the speaker is not strongly stating the situation as an objective fact. Instead, they are saying:

If you feel / if you think storage is insufficient, then...

This can sound a bit softer or more considerate, since the speaker is basing the suggestion on the listener’s judgment.

なら means if, but it often has the nuance of if that’s the case or if you say/think that.

In this sentence:

• 収納が足りないと思うなら
  = if you think there isn’t enough storage

It connects the idea before it with the suggestion after it.

A helpful way to think about なら:

• It often responds to a situation, assumption, or topic already being discussed.
• It can feel conversational, like if that’s how it is...

So this sentence has a natural spoken flow:

• If you think storage is lacking, how about buying a big bookshelf?

Yes. 〜たらどう is a very common way to make a suggestion:

• 買ったらどう？ = How about buying one? / Why don’t you buy one?

It is short for something like:

• 買ったらどうですか
• 買ったらどうだ

In casual conversation, the ending is often shortened to just どう？

So:

• 大きい本棚を買ったらどう？
  = How about buying a big bookshelf?

This pattern is common:

• 休んだらどう？ = Why don’t you rest?
• 先生に聞いたらどう？ = How about asking the teacher?

Usually it is a fairly natural, conversational suggestion. It can sound like:

• Why don’t you... ?
• How about... ?

The tone depends a lot on context and voice.

In many situations, 〜たらどう？ is friendly and practical. But compared with softer forms like 〜たほうがいい or 〜ませんか, it can sometimes sound a little direct if used with the wrong tone.

Here it sounds normal because the speaker is offering a practical idea in casual speech.

Both 大きい本棚 and 大きな本棚 are possible.

• 大きい is an i-adjective
• 大きな is a special prenominal form used only before nouns

Both mean big, but there can be a slight nuance difference:

• 大きい本棚 = straightforward, neutral big bookshelf
• 大きな本棚 = sometimes slightly more descriptive, literary, or impressionistic

In everyday speech, 大きい本棚 is very common and perfectly natural.

Yes, 本棚 literally means bookshelf or bookcase:

• 本 = book
• 棚 = shelf

In this sentence, the idea is that if storage is insufficient, buying a large bookshelf could help create more space for books or other items.

Depending on context, a Japanese speaker might also use other words such as:

• 棚 = shelf
• 収納棚 = storage shelf/cabinet
• ラック = rack/shelving unit

But 本棚 is a very natural choice if books are part of the storage problem.

Yes, this sentence is casual.

Casual version:

• 収納が足りないと思うなら、大きい本棚を買ったらどう？

A polite version could be:

• 収納が足りないと思うなら、大きい本棚を買ったらどうですか。

Or a slightly softer suggestion:

• 収納が足りないと思うなら、大きい本棚を買ってみたらどうですか。

In casual Japanese, it is very common to drop です / ます and end with どう？

In this sentence, it is understood to be the listener: if you think.

Japanese often leaves subjects unstated when they are obvious from context. So even though あなたが is not written, the meaning is naturally:

• If you think there isn’t enough storage...

This is one of the most common things English speakers need to get used to in Japanese: the subject is often omitted when it can be inferred.

Yes, absolutely. That would be very natural too.

Compare:

• 収納が足りないなら...
  = if there isn’t enough storage...
• 収納が足りないと思うなら...
  = if you think there isn’t enough storage...

The second version is a little softer and more subjective. It frames the situation as the listener’s opinion or feeling rather than a firm statement of fact.

So both are correct, but the version with と思う sounds a bit more considerate or less blunt.