kono tana nara, hon wo takusan okemasu.

この is a determiner, so it must come before a noun:

• この棚 = this shelf

By contrast, これ is a standalone pronoun:

• これ = this

So:

• この棚 = correct
• これ棚 = incorrect
• これ would mean this, but it would not specifically say shelf

Here, なら attaches to この棚 and gives a meaning like:

• as for this shelf
• if it's this shelf
• when it comes to this shelf

So この棚なら suggests something like:

• This shelf should work
• If we're talking about this shelf, then...
• This shelf, at least, can handle it

It often carries a feeling of contrast with other options.

Both can be translated similarly in some contexts, but the nuance is different.

• この棚は = a plain topic: As for this shelf...
• この棚なら = a conditional/contrastive topic: If it's this shelf... / This shelf, at least...

So なら often implies:

• maybe other shelves are not suitable
• this shelf is being picked out from several choices
• the speaker is making a judgment based on that choice

In this sentence, なら sounds more like This shelf, unlike the others, can hold a lot of books.

That is a very natural question.

With 置く, the place is often marked by に:

• 棚に本を置く = put books on the shelf

But in this sentence, この棚 is not being marked mainly as the location. It is being presented as the topic/condition with なら.

So:

• この棚なら、本をたくさん置けます。
  = If it's this shelf / As for this shelf, you can place lots of books.

You could also say:

• この棚になら、本をたくさん置けます。

That version makes the location role more explicit. The original sentence is still natural; it just focuses more on this shelf as the option under discussion.

置けます is the polite potential form of 置く.

• dictionary form: 置く = to place / to put
• potential form: 置ける = can place / can put
• polite potential form: 置けます

So the sentence uses polite speech.

A plain-form version would be:

• この棚なら、本をたくさん置ける。

Because 本 is still the thing being placed:

• 本を置く = place books
• 本を置けます = can place books

Japanese potential expressions sometimes use が instead of を, and learners often notice that. That is true:

• 日本語が話せる
• 魚が食べられる

But in modern Japanese, を is also very common with potential forms, especially when the verb still feels strongly like an action someone is able to perform.

So 本をたくさん置けます is completely natural.

Here たくさん tells you the quantity of books that can be placed.

So:

• 本をたくさん置けます = can place a lot of books

It works adverbially here, modifying the whole action.

You can also say:

• たくさんの本を置けます

That also means can place a lot of books.

A small nuance:

• 本をたくさん置けます focuses a bit more on placing books in large quantity
• たくさんの本を置けます treats a lot of books more directly as a noun phrase

Both are natural.

Japanese often leaves the subject unstated when it is clear from context.

So this sentence could imply:

• you can place a lot of books
• we can place a lot of books
• one can place a lot of books

In natural English, though, we often translate it as:

• This shelf can hold a lot of books

That sounds smoother in English, even though the Japanese grammar is built around being able to place books rather than the shelf itself being the grammatical subject.

So the English meaning may sound slightly less literal than the Japanese structure.

Yes.

The politeness comes from 置けます.

Compare:

• polite: 置けます
• plain: 置ける

So:

• この棚なら、本をたくさん置けます。 = polite
• この棚なら、本をたくさん置ける。 = plain/casual

The rest of the sentence is neutral; the main politeness marker is the verb ending.

The comma shows a natural pause after この棚なら.

It helps separate:

• the topic/condition: この棚なら
• the main statement: 本をたくさん置けます

So it matches the way the sentence is often spoken:

• この棚なら、 / 本をたくさん置けます。

It is not absolutely required every time, but it is very common and makes the sentence easier to read.

Very often, yes.

Using なら frequently suggests something like:

• other shelves may not work
• this shelf is the suitable one
• among the available choices, this one is good enough

So the nuance can be:

• This shelf, at least, can hold lots of books.
• If we use this shelf, we can put lots of books on it.

That contrastive feeling is one of the main reasons なら is used here instead of just は.