Learn Hardcore Japanese: nihongo no bunpou ga wakatte kuru to, nihongo de hanasuno ga masumasu tanosiku narimasu. - As you come to understand Japanese grammar, speaking in Japanese becomes more and more fun.

In written, technical contexts you can sometimes see things like 日本語文法, but in normal, natural Japanese, you generally use A の B for "B of A."

Why is が used after 文法 in 日本語の文法が分かってくる instead of は?

With the verb 分かる, the thing that is understood is usually marked by が:

文法が分かる = "I understand the grammar."

So:

日本語の文法が分かる = "I understand Japanese grammar."
日本語の文法が分かってくる = "I’m coming to understand Japanese grammar (gradually)."

You could say 日本語の文法は分かってくる if you want to contrast it with something else (e.g., "As for Japanese grammar, I'm starting to understand it (but something else I don't)").

Default, neutral statement of what is understood → が.

What exactly does 分かってくる mean, compared to just 分かる?

分かってくる is て-form + くる, and it often expresses a change that builds up over time and reaches the present.

分かる: "to understand" (simple fact)
分かってくる: "to come to understand," "to gradually start understanding," "to begin to get it"

Nuance:

日本語の文法が分かる
→ I understand Japanese grammar. (static fact)
日本語の文法が分かってくる
→ I am starting to understand / I have been increasingly understanding Japanese grammar (as time goes on).

It emphasizes the process and gradual change, not just the end result.

What does と do in 日本語の文法が分かってくると? Is it "and"?

Here と is a conditional particle, meaning roughly "when" or "once", not "and."

日本語の文法が分かってくると、〜
→ "When you come to understand Japanese grammar, 〜"
→ "Once you start to understand Japanese grammar, 〜"

This type of と expresses a natural result or automatic consequence:

春になると、暖かくなります。
"When it becomes spring, it gets warm."

Same pattern in your sentence:
Understanding grammar → naturally leads to speaking becoming more fun.

What does で mean in 日本語で話すの?

で here marks the means / medium used to do something:

日本語で話す = "to speak in Japanese"
英語で書く = "to write in English"
車で行く = "to go by car"

So 日本語で = "in Japanese (as the language you use)."

Why is it 話すの and not just 話す or 話すこと?

話すの is using の to nominalize the verb: turn "to speak" into "speaking."

話す = to speak
話すの = speaking (the act of speaking)
話すこと = speaking (also a nominalization)

All three are grammatically possible in some contexts, but:

〜の is more natural and casual/neutral in everyday speech.
〜こと sounds a bit more formal or written, or is used in specific grammar patterns.

Here, 話すのが楽しくなります =
"Speaking (in Japanese) becomes fun."

Saying just 話すが楽しくなります would be ungrammatical; you need の (or こと) to turn 話す into a noun phrase.

Why is there another が after 話すの in 日本語で話すのがますます楽しくなります?

This が is marking the subject of the verb なります:

日本語で話すの = speaking in Japanese
が = subject marker
ますます楽しくなります = becomes more and more fun

So literally:

"Speaking in Japanese becomes more and more fun."

Having two が in the same sentence is fine because they belong to different clauses:

日本語の文法が分かってくると、
- 日本語の文法が = subject of 分かってくる
日本語で話すのがますます楽しくなります。
- 日本語で話すのが = subject of 楽しくなります

They’re separate pieces joined by the conditional と.

Why is it 楽しく and not 楽しい before なります?

With い-adjectives and なる ("to become"), you normally use the adverbial (連用形) form of the adjective:

楽しい (fun) → 楽しくなる (to become fun)
早い (fast) → 早くなる (to become fast / faster)
暑い (hot) → 暑くなる (to become hot)

So:

楽しくなります = "becomes fun," "gets more enjoyable."

Using 楽しいなります is ungrammatical. It must be 楽しくなる.

What does ますます mean in ますます楽しくなります?

ますます is an adverb meaning:

"more and more"
"increasingly"
"even more"

So:

ますます楽しくなります
= "It becomes more and more fun."
= "It gets increasingly enjoyable."

It emphasizes that the fun keeps increasing over time, not just "becomes fun once."

Why is the sentence in non-past (分かってくる, なります) if the English meaning is about the future ("will become")?

Japanese non-past (the form often called "present") covers both:

present/habitual: "is / does"
and general/future: "will / when [something happens]"

In this type of general statement:

日本語の文法が分かってくると、〜楽しくなります。

it describes a general truth or tendency:

Whenever / when you come to understand Japanese grammar, speaking in Japanese (generally) becomes more and more fun.

English often uses "will" in this kind of general statement, but Japanese just uses the plain non-past. So なります can be understood as "becomes / will become" depending on context.