Learn Hardcore Japanese: tosyokan no zisyuusitu nara, nagaku syuutyuuryoku wo tamoteru to omoimasu. - In the library’s self-study room, I think I can maintain my concentration for a long time.

図書館の自習室なら、長く集中力を保てると思います。
→ As for the library’s study room / If it’s the library’s study room, I think (one) can keep concentration for a long time.

Comparisons:

図書館の自習室は、長く集中力を保てると思います。
This sounds more like a plain statement of fact about the library study room. Less conditional, more like you’re simply describing it.
図書館の自習室で、長く集中力を保てると思います。
This uses で as a location marker (“at the library study room”), but it loses the “when it comes to / if we’re talking about that” nuance. It’s grammatically OK, but it sounds more neutral and factual, less like “among various options, that one works”.

So なら here suggests something like:
“In the case of the library’s study room (as opposed to other places), you can keep your concentration for a long time.”

Why is it 図書館の自習室 and not just 図書館?

図書館の自習室 literally means “the library’s self-study room” / “the study room in the library.”

If you said:

図書館なら、長く集中力を保てると思います。

this would mean:

If it’s the *library (as a place), I think I can keep my concentration for a long time.*

That suggests anywhere in the library—the lobby, the stacks, a table area, etc.

By saying 図書館の自習室, you narrow it down to a specific area designed for studying, often a quiet room with individual desks. So the sentence is more specific:

If it’s the library’s *study room (that particular quiet room), I think I can keep my concentration for a long time.*

Why is it 長く and not 長い before 集中力を保てる?

長い is the adjective form (“long”).
長く is its adverb form, which modifies verbs.

In the sentence:

長く集中力を保てる

長く modifies 保てる (can maintain), not 集中力.

So:

長い集中力 would mean “long concentration” (describing the noun).
長く集中力を保てる means “can keep (maintain) concentration for a long time.”

In English we’d use a phrase (“for a long time”), but in Japanese this is often just the adverb form of the adjective:
長い → 長く (for a long [time])
速い → 速く (quickly/fast)
強い → 強く (strongly)

What is 保てる? What’s the base verb and what form is this?

The base verb is 保つ (たもつ), which means “to keep / to maintain / to preserve.”

保てる is the potential form of 保つ, meaning “can keep / can maintain.”

Pattern for potential form (Group 1 verbs / godan verbs):

～つ → ～てる
- 持つ → 持てる (can hold)
- 立つ → 立てる (can stand [something up])
- 保つ → 保てる (can maintain)

So:

集中力を保つ = “to maintain one’s concentration”
集中力を保てる = “can maintain one’s concentration”

I learned potential verbs usually use が instead of を. Why is it 集中力を保てる and not 集中力が保てる?

You probably learned a rule like:
> With potential verbs, the “object” often takes が instead of を.

That’s true in many cases:

日本語が話せます。 – I can speak Japanese.

But using を is also possible with many potential verbs, especially when:

The verb in its non-potential form naturally takes を, and
You want to keep a more “direct object” kind of feel.

For 保つ:

Normal form: 集中力を保つ – to maintain concentration
Potential form:
- 集中力が保てる – concentration can be maintained
- 集中力を保てる – (I/you/we) can maintain concentration

Nuance:

集中力が保てる focuses a bit more on 集中力 itself:
“Concentration can be maintained (in that environment).”
集中力を保てる feels a bit more like “(someone) can keep (their) concentration.”

Both are grammatically correct. Many native speakers use を naturally in this expression.

What does と do in 保てると思います? Is it “and”?

Here と is not “and.” It’s the quotative particle: it marks what is being thought, said, felt, heard, etc.

〜と思います = “I think that 〜”
〜と言いました = “(someone) said that 〜”
〜と感じます = “I feel that 〜”

In the sentence:

長く集中力を保てる
is the content of the thought.
長く集中力を保てると思います。
literally: “I think ‘I can keep my concentration for a long time’.”

So と here is like quotation marks in English, or like “that” in “I think that I can…”

Why is it 保てると思います (plain + polite)? Can I say 保てますと思います?

In Japanese, when you attach と思います to a clause, that clause is normally in the plain form, even in polite speech:

行けると思います。 ✅
高いと思います。 ✅
静かだと思います。 ✅

Using ます before と思います is generally unnatural:

行けますと思います。 ❌ (sounds wrong/foreign)
保てますと思います。 ❌

So the correct pattern is:

[plain form] + と思います

In your sentence:

保てる is the plain potential form.
思います is polite.

This mix (plain inside, polite outside) is standard and correct:

長く集中力を保てると思います。 ✅

Could you also say 図書館の自習室だと、長く集中力を保てると思います。? What’s the difference between なら and だと here?

Yes, you can say:

図書館の自習室だと、長く集中力を保てると思います。

It’s grammatically fine and understandable.

Nuance difference:

図書館の自習室なら
→ “As for / if it’s the library study room (as a condition or choice among others), I think you can keep your concentration…”
This has a somewhat hypothetical / contrastive feel: in that case (unlike others).
図書館の自習室だと
→ “If it is the library study room, (then) you can keep your concentration…”
This sounds a bit more neutral and factual, like describing what happens when that condition is true.

They overlap a lot in spoken Japanese. なら is often preferred for this “when it comes to X / in the case of X” sense, especially when you’re recommending or contrasting that option with others.

Who is the subject in 長く集中力を保てると思います? Does it mean “I can keep my concentration” or “one can keep their concentration”?

Japanese often omits the subject when it’s clear from context.

In isolation, 長く集中力を保てると思います could be interpreted as:

I can keep my concentration for a long time.
You can keep your concentration for a long time.
People / one / we can keep concentration for a long time.

Which one is intended depends on the conversation:

If you’re talking about your own experience, it’s likely “I”:
- When I’m in the library study room, I can keep my concentration for a long time.
If you’re giving advice or a recommendation, it can be more general/you/we:
- If it’s the library study room, you can keep your concentration for a long time (so you should try it).

This kind of subjectless sentence is very natural in Japanese.