Learn Hardcore Japanese: watasi ha nihongo no benkyou wo taisetu ni simasu.

it sounds like you are contrasting yourself with someone else or emphasizing that it is I (and not someone else) who values this. It’s a more specific, focused, or contrastive nuance.

In most neutral, self-introduction-style sentences, は is the natural choice.

Why is there の between 日本語 and 勉強? Why not just say 日本語勉強?

の links two nouns, and often works like “of” in English.

日本語 = Japanese (language)
勉強 = study, studying
日本語の勉強 = the study of Japanese / Japanese study

Without の, 日本語勉強 sounds unnatural; Japanese usually needs の to connect nouns in this kind of “X of Y” relationship.

So 日本語の勉強 literally means “Japanese’s study” → more naturally “studying Japanese.”

What role does を play in 日本語の勉強を here?

を marks the direct object of the verb (or verb-like expression).

In this sentence, the whole phrase 日本語の勉強 (“study of Japanese”) is what you are doing something to.
The predicate is 大切にします, which literally means “to treat something as important / cherish something.”

So:

日本語の勉強を大切にします。
→ “(I) treat my Japanese studies as important.”
→ “(I) value my Japanese studies.”

を shows that 日本語の勉強 is the thing being valued/cherished.

Why is there に after 大切? What does 大切に mean exactly?

大切 by itself is a na-adjective meaning “important, precious.”

大切な本 = an important / precious book
これは大切です。 = This is important.

When you add に to a na-adjective, you can turn it into an adverbial phrase (showing the manner or attitude of an action):

大切にする literally: “to do (something) in an ‘important/precious’ way” → “to treat something as important,” “to cherish / value something.”

So:

大切にします = “(I) will value it / treat it carefully / cherish it.”
日本語の勉強を大切にします = “I will value my Japanese studies.”

Think of 大切に as “with importance / carefully / reverently” modifying します.

What’s the difference between 大切にします and just 大切です?

They have related meanings but different grammar and nuance.

大切です
- 大切 is a na-adjective.
- Saying Xは大切です means “X is important.”
- Focus: describes a state.
Example:
- 日本語の勉強は大切です。
  → “Studying Japanese is important.”
大切にします
- This is a verb phrase: “to treat (something) as important,” “to cherish.”
- Structure: (object) を大切にします
- Focus: describes an action / attitude you choose to take.
Example:
- 私は日本語の勉強を大切にします。
  → “I (will) value / cherish my Japanese studies.”
  → “I make my Japanese studies a priority.”

So 大切です is more like stating a fact, while 大切にします expresses your intention or ongoing attitude about how you will treat that thing.

Is 大切にします in present or future tense? How should I understand the time?

Japanese polite present form (〜ます) covers present, habitual, and future meanings, depending on context.

大切にします can mean:

Present / habitual:
→ “I value (it).” / “I (generally) treat it as important.”
Future / intention:
→ “I will value (it).” / “I’m going to treat it as important (from now on).”

In isolation, 私は日本語の勉強を大切にします。 is usually understood as a general attitude or intention:
“I value my Japanese studies / I will make my Japanese studies important (in my life).”

Can I drop 私 and just say 日本語の勉強を大切にします?

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

日本語の勉強を大切にします。
→ Usually understood as “I value my Japanese studies,” if you’re talking about yourself.

You might keep 私 in situations like:

Self-introductions, where you want to be extra clear or formal.
When contrasting with someone else: 私は〜, 彼は〜.

But in everyday conversation, leaving out 私 is completely natural.

How flexible is the word order? Can I say 私は日本語の勉強を大切にします vs 日本語の勉強を私は大切にします?

Japanese word order is more flexible than English because particles show each word’s role.

The most neutral, common order is:

私は日本語の勉強を大切にします。

You can say:

日本語の勉強を私は大切にします。

This puts some extra focus on 私 (“I, at least, value my Japanese studies”), often implying a contrast with someone else, or emphasizing “me” as the one with this attitude.

As a beginner, it’s safest to stick with:

[topic] は [object] を [predicate]
→ 私は日本語の勉強を大切にします。

How is 大切にします different from just saying 勉強します or 一生懸命勉強します?

They focus on different aspects:

勉強します
- “I study / I will study.”
- Focus: the action of studying itself.
一生懸命勉強します
- “I will study very hard / do my best in studying.”
- 一生懸命 = “with all one’s might.”
- Focus: how hard or seriously you study.
勉強を大切にします
- “I will value / cherish my studies.”
- Focus: your attitude about study: you make it a priority, treat it as something important in your life.

So:

日本語を一生懸命勉強します。
→ “I will study Japanese very hard.”
日本語の勉強を大切にします。
→ “I will value my Japanese studies (make them important / not neglect them).”

You can even combine ideas:

日本語の勉強を大切にして、一生懸命勉強します。
→ “I will value my Japanese studies and work very hard at them.”

Is there a difference between 大切にする and 大事にする?

They are very close in meaning and often interchangeable:

大切にする
大事にする

Both mean: “to treat something as important / to cherish / to value.”

Nuance (very subtle, and often not important for learners):

大切 sometimes feels slightly more formal or “serious,” and is very common in set phrases like 命を大切にする (“to value life”).
大事 is also very common, maybe a bit more colloquial in some uses, and often appears in casual speech.

For your sentence, both are natural:

私は日本語の勉強を大切にします。
私は日本語の勉強を大事にします。

For most beginner and intermediate purposes, you can treat them as near-synonyms here.