Learn Hardcore Japanese: watasi ha itinen nihongo wo benkyousimasita.

は (wa) marks the topic: “As for me…”. It frames what the sentence is about, not necessarily the grammatical subject. Using が (ga) would mark “I” as the specific subject and adds focus/contrast like “It was I (not someone else) who…”. Compare:

私は一年日本語を勉強しました。 As for me, I studied Japanese for one year. (neutral)
私が一年日本語を勉強しました。 It was I who studied Japanese for one year. (contrastive/emphatic)

Can I drop 私?

Yes. Subjects are often omitted when clear from context. 一年日本語を勉強しました。 is very natural. You can also say 日本語を一年勉強しました。

Why is 一年 placed before 日本語? Where do time expressions go?

Time expressions (duration, point-in-time, frequency) commonly appear early in the clause, often before the object. So 私は一年日本語を勉強しました is typical. You can also say 私は日本語を一年勉強しました—both are fine. Time phrases are flexible but usually don’t come after the verb.

Do I need a particle after 一年 to mean “for one year”? Why is there no に?

No particle is needed for duration. Bare duration nouns (e.g., 一年, 三時間, 五日) function adverbially to mean “for X time.” Don’t use に here; 一年に means “per year” (frequency), not “for one year.”

What’s the difference between 一年, 一年間, and 一年の間?

All can mean “for one year.”

一年: plain, neutral.
一年間: slightly emphasizes the span; feels a touch more explicit.
一年の間: literally “during the one-year period,” a bit more formal/explicit.
All three work in: （私は）一年(間/の間)日本語を勉強しました。

How do I say “about/around one year”?

Add くらい/ぐらい or ほど after the duration:

一年くらい日本語を勉強しました。
一年ほど日本語を勉強しました。
Both mean “about one year.”

How do I say “one year ago” vs “for one year”?

“One year ago” = 一年前: 一年前、日本語を勉強しました。 (I studied Japanese a year ago.)
“For one year” = 一年(間): 一年間、日本語を勉強しました。 (I studied Japanese for a year.)

What does を do after 日本語?

を (o) marks the direct object of a transitive verb. 勉強する (to study) is transitive, so what you study (日本語) takes を: 日本語を勉強しました。 Not 日本語が here.

Why can’t I use です here? Is 勉強しましたです okay?

No. です attaches to nouns and adjectives, not to verbs in their polite forms. The verb ending 〜ます/〜ました already provides politeness. 勉強しましたです is ungrammatical. Just say 勉強しました.

What’s the nuance difference between 勉強しました, 勉強していました, and 勉強しています?

勉強しました: completed action in the past (I studied).
勉強していました: was studying (ongoing in a past timeframe; often implies not anymore).
勉強しています: is/has been studying (currently ongoing or habitual). With a duration like 一年間, it usually means “have been studying for one year (and still am).”

If I’m still studying now, how should I say it?

Use the progressive:

日本語を一年間勉強しています。 (I have been studying Japanese for a year and still am.) Other natural options:
日本語を勉強し始めて一年になります。 (It’s been a year since I started studying Japanese.)
一年間、日本語を勉強してきました。 (I’ve been studying up to now; nuance of “leading up to the present.”)

Can I say 日本語は一年勉強しました? How is it different from 日本語を?

Yes, but it changes the focus. 日本語は一年勉強しました makes 日本語 the topic, often with a contrastive feel (“As for Japanese, I studied it for one year [as opposed to something else]”). 日本語を一年勉強しました is the neutral “I studied Japanese for one year.”

Is the word order flexible? Which versions are natural?

Natural variants include:

私は一年日本語を勉強しました。
私は日本語を一年勉強しました。
一年、日本語を勉強しました。 Avoid inserting the duration inside the object phrase in a way that splits noun+particle incorrectly (e.g., don’t do 日本語一年を…). Keep 日本語を together, and place 一年(間) as its own adverbial chunk.

Do I need spaces between words in Japanese?

No. Japanese is normally written without spaces. As a finished sentence you’d write: 私は一年日本語を勉強しました。 Spaces are often added in teaching materials for clarity.

How do I pronounce the particles は and を here?

は is pronounced “wa” when used as the topic particle (so 私は = “watashi wa”).
を is pronounced “o” in standard Japanese (so 日本語を = “nihongo o”).
Full reading: わたしはいちねんにほんごをべんきょうしました。

Is 日本語 always read にほんご, or can it be にっぽんご?

In this context, the standard reading is にほんご (Nihongo). にっぽんご is rare and not used in ordinary conversation for “the Japanese language.”

What’s the difference between 勉強する, 習う, and 学ぶ?

勉強する: to study (broad, covers self-study and classes).
習う: to learn from someone; to take lessons (often with 〜を[人]に習う).
学ぶ: to learn/study in a more academic, formal, or deep sense.
Examples: 日本語を勉強しました。/ 日本語を先生に習いました。/ 日本語を学びました。

Can I omit を in casual speech?

In fast/casual speech, some speakers drop を before certain verbs: 日本語(を)勉強しました. It’s common in speech but less so in writing and can be risky for learners. Keeping を is safest and always correct.

How do I say “It took me one year to learn Japanese,” as opposed to “I studied for one year”?

Use で to express the time required to reach a result:

一年で日本語を話せるようになりました。 (I became able to speak Japanese in one year.)
Compare with duration of activity: 一年間日本語を勉強しました。 (I studied Japanese for one year.)

How can I add emphasis like “as long as one year” or “only one year”?

Emphatic “as long as”: 一年間も日本語を勉強しました。
Limiting “only”: 一年間しか日本語を勉強しませんでした。 (Note the negative with しか.)

When would I use 一年に?

一年に means “per year” (frequency), not duration. For example: 一年に三回帰国します。 (I go back to my country three times a year.) Don’t use 一年に to mean “for one year.”