Learn Hardcore Japanese: koko nara, nihongo wo benkyou dekiru.

ここで日本語を勉強できる = You can study Japanese at this place (simple statement of location).
ここなら日本語を勉強できる = If it’s here (as opposed to somewhere else), you can study Japanese / Here, at least, you can study Japanese.

So なら adds:

a sense of condition: “if X is the case…”
or contrast/limitation: “speaking of X (and maybe not other places), …”

It often implies that other places might not be OK, but here is OK.

What is the subject of this sentence? There’s no I or you.

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

ここなら、日本語を勉強できる。 could mean, depending on context:

(私は) ここなら、日本語を勉強できる。
→ I can study Japanese here (if it’s here).
(あなたは) ここなら、日本語を勉強できる。
→ You can study Japanese here.
(ここでは人が) 日本語を勉強できる。
→ People can study Japanese here.

In real conversations, who “can” do it is usually understood from the situation, so the pronoun is dropped.

Why is を used after 日本語? Isn’t 日本語 just a language?

In this sentence, 日本語 (Japanese) is the object of the action 勉強 (study), so を marks it as what is being studied.

日本語 = Japanese (language)
勉強 = study (a noun)
日本語を勉強する = to study Japanese
日本語を勉強できる = can study Japanese

So を here works exactly like “study Japanese” in English, where “Japanese” is the direct object of “study”.

What exactly does 勉強できる mean, and how is it formed?

勉強できる (benkyou dekiru) literally means “to be able to study”, i.e., “can study”.

Structure:

勉強 = “study” (a noun)
できる = “to be able to do / can be done”

When you put a noun + できる, it often means “can do [that noun]”:

勉強できる = can study
練習できる = can practice
予約できる = can make a reservation

So 日本語を勉強できる = (someone) can do the action of studying Japanese.

Could we also say 日本語が勉強できる instead of 日本語を勉強できる?

Yes, 日本語が勉強できる is also correct and quite common.

With potential expressions (like できる or the potential form of verbs), が often marks the thing that can be done:

日本語が勉強できる
日本語を勉強できる

Both are used. Rough nuances (often subtle and context‑dependent):

が tends to emphasize “Japanese is what I can study”.
を feels a bit more like an ordinary object marker carried over from 日本語を勉強する.

In casual speech, many speakers freely use を here; textbooks often introduce が as the default with potential verbs.

Why is 勉強できる used instead of 勉強することができる? Are they different?

勉強できる and 勉強することができる mean essentially the same thing: “can study”.

日本語を勉強できる。
日本語を勉強することができる。

Differences:

勉強できる: shorter, more natural in everyday speech.
勉強することができる: more formal / stiff / written, often used in explanations, official notices, manuals, etc.

For normal conversation, 勉強できる is the default choice.

What is the difference between ここ, ここで, and ここなら in this kind of sentence?

Using your example meaning:

ここで日本語を勉強できる。
→ You can study Japanese *at this place.*
Just states the place where the action happens.
ここなら日本語を勉強できる。
→ If it’s here, you can study Japanese.
Implies: maybe other places are not good, but here is okay. Slight contrast / condition.
ここでなら日本語を勉強できる。
→ combines both:
- ここで (at this place)
- なら (if / as for)
  Nuance: At this particular place, at least, you can study Japanese (though maybe not elsewhere).
  Often used to emphasize “this specific place” in contrast to other places.

So なら adds a sense of “if it’s X” or “speaking of X (and maybe not other options)”.

Is ここなら、日本語を勉強できます。 also correct? What’s the difference with できる?

Yes, ここなら、日本語を勉強できます。 is perfectly correct.

できる = plain / casual form
できます = polite -ます form

Choose based on who you’re talking to:

With friends, family, people close to you:
→ ここなら、日本語を勉強できる。
With teachers, strangers, customers, in class, etc.:
→ ここなら、日本語を勉強できます。

The meaning (can study) is the same; only politeness level changes.

Why are there spaces between the words in ここ　なら、　日本語　を　勉強　できる。? Do Japanese people write like that?

In normal Japanese writing, there are usually no spaces between words:

Natural: ここなら、日本語を勉強できる。

The version with spaces is probably written for learners, to help you see the word boundaries:

ここ｜なら、｜日本語｜を｜勉強｜できる。

Native texts (books, manga, newspapers, etc.) almost never put spaces between words, only:

punctuation like 、 and 。
line breaks
sometimes special spacing for style, but not regularly between every word.

Where can I put なら in the sentence? Can I move it somewhere else?

なら usually attaches directly to the phrase being contrasted or conditioned.

Here, the “condition” is the place (ここ), so we say:

ここなら、日本語を勉強できる。 ✓

If you move なら away from ここ, it becomes unnatural or changes the meaning:

✗ ここ日本語ならを勉強できる。 (ungrammatical)
✗ ここなら日本語を勉強できる is fine (no comma is OK).
日本語なら、ここで勉強できる。
→ Now the thing being contrasted is 日本語:
If it’s Japanese (as opposed to some other subject), you can study it here.

So: attach なら right after the thing you want to say “if it’s X / as for X” about. In your sentence, that’s the word ここ.