Learn Hardcore Japanese: konsyuu ha mainiti tosyokan de nihongo no kaiwa wo rensyuusurukoto ni simasu. - This week I will make it a rule to practice Japanese conversation at the library every day.

If you said 今週に, it would sound like you’re marking a specific time point (“at/in this week”), which is not natural here. You’re not pinpointing a time; you’re framing what you’re going to do during this week in general, so Japanese prefers a topic with は.

In short:

今週は… = “This week, (I will) …” (topic)
Not 今週に…, because that would treat it like a precise time adverb, which doesn’t fit the broad “this week as a whole” nuance here.

Why does 毎日 have no particle? Can I say 毎日に or 毎日を?

Time expressions like 毎日 (every day), 明日 (tomorrow), 昨日 (yesterday), 来週 (next week) often appear without a particle when they simply tell you when something happens.

So:

毎日図書館で… = “(do something) at the library every day”

Saying 毎日に or 毎日を here would be unnatural.
You can add particles to time words in special situations (e.g. 毎日が忙しい “every day is busy”), but for “do X every day”, 毎日 + verb without a particle is the normal pattern.

So 今週は毎日図書館で… = “This week, (I will) do it at the library every day.”

What’s the difference between 図書館で and 図書館に here?

図書館で uses で, which marks the place where an action occurs.
- 図書館で練習する = “practice at/in the library.”
図書館に usually marks:
- a destination (“to the library”)
- or sometimes a location where something/someone exists with いる / ある (“at the library”).

In this sentence we care about where the practicing happens, so で is correct:

❌ 図書館に日本語の会話を練習する – sounds wrong/odd.
✅ 図書館で日本語の会話を練習する – practice Japanese conversation at the library.

Why is it 日本語の会話 and not something like 日本語会話 or 日本語で会話?

Here, の links two nouns:

日本語 (Japanese language)
会話 (conversation)

So 日本語の会話 literally means “conversation of Japanese (language)”, i.e. Japanese conversation.

Alternatives and nuances:

日本語の会話
- Grammatically: noun + の
  - noun
- Very clear: “Japanese conversation” (as a type of conversation).
日本語会話
- A compound noun; you’ll see it in course names, textbooks, etc.
- In everyday speech, 日本語の会話 is more neutral and natural.
日本語で会話
- Uses で to mean “in/by means of (Japanese)”.
- 日本語で会話する = “have a conversation in Japanese.”
- Slightly different structure: here 日本語で modifies the manner of the conversation, not naming the type of conversation as a noun phrase.

In your sentence, we’re treating “Japanese conversation” as the object of practice, so 日本語の会話を練習する is a natural way to say “practice Japanese conversation.”

Why is 会話 marked with を? I thought 練習する already meant “to practice”.

練習する is a verb that usually takes a direct object with を:

ピアノを練習する = practice the piano
漢字を練習する = practice kanji
会話を練習する = practice conversation

So 会話 is the thing being practiced, and を marks it as the direct object:

日本語の会話を練習する
= practice Japanese conversation.

You could also say:

日本語の会話の練習をする
(literally “do practice of Japanese conversation”), which is close in meaning, just a different structure.

In this sentence the simpler pattern [object] を練習する is being used.

What is すること doing here? Why not just say 日本語の会話を練習します?

こと turns the verb phrase 練習する into a noun-like unit (a nominalization):

練習する = to practice
練習すること = “the act of practicing” / “practicing (as a thing)”

This is needed because of the pattern 〜ことにする, which literally means “to make it (into) the thing that ~”, and idiomatically becomes:

〜することにする = “decide to do ~”

So:

日本語の会話を練習します
= “I (will) practice Japanese conversation.” (simple statement)
日本語の会話を練習することにします
= “I (have) decided to practice Japanese conversation.”
(focuses on the decision, not just the action)

You can’t say 練習するにします; the pattern specifically requires verb (plain form) + こと + にする.

What exactly does 〜ことにします mean? How is it different from just using the verb?

〜ことにする is a set phrase that means:

“to decide to ~”
or “to make it a rule/plan to ~”

Structure:

[plain-form verb] + ことにする
- 行くことにする = decide to go
- 勉強することにする = decide to study
- 日本語の会話を練習することにする = decide to practice Japanese conversation

Nuance compared with just the verb:

練習します
- Simple statement of intention/habit: “I will practice / I practice.”
練習することにします
- Emphasizes the act of choosing/deciding to do it. Often used when you’ve just made the decision or are announcing your plan.

So the full sentence is not just “I practice every day”, but more like “This week, I will go with practicing Japanese conversation at the library every day / I’ve decided that that’s what I’ll do.”

What’s the difference between 〜ことにします and 〜ことにしました?

Both come from the same pattern 〜ことにする (decide to ~), but the tense changes the nuance:

〜ことにします (non-past polite)
- Often used when you are just now deciding or announcing your decision.
- Feels like: “I think I’ll do ~ / I’ll go with ~.”
〜ことにしました (past polite)
- Indicates the decision was made already (even if just a moment ago).
- Feels like: “I (have) decided to ~.”

In your sentence:

…練習することにします。
→ sounds like you’re deciding right now, maybe in front of someone, or stating your fresh plan.

If you said:

…練習することにしました。
→ “I decided (earlier) to practice…”; you’re reporting a decision you’ve already made.

How flexible is the word order here? Can I move 毎日 or 今週は around?

Japanese word order is relatively flexible, especially with adverbs and time expressions. Common natural variants include:

今週は毎日図書館で日本語の会話を練習することにします。
今週は図書館で毎日日本語の会話を練習することにします。
毎日図書館で日本語の会話を練習することにします。今週は。 (bit more colloquial, with 今週は added for emphasis at the end)

General points:

は-marked topics (like 今週は) usually appear near the beginning.
Adverbs like 毎日 can appear before or after the topic, as long as the meaning stays clear.
The verb phrase at the end (…練習することにします) is the part least likely to move.

All of these keep the basic meaning; the differences are in emphasis and naturalness, not in grammar correctness.

Who is the subject of this sentence? There’s no “I” written in Japanese.

Japanese often omits the subject when it’s clear from context. In this sentence:

今週は毎日図書館で日本語の会話を練習することにします。

The implied subject is “I” (or “we”), because:

The verb 〜ことにします describes a personal decision.
In a normal conversation about your own plans, the default assumption is that you are the one deciding.

If you wanted to show the subject explicitly, you could say:

私は今週は毎日図書館で日本語の会話を練習することにします。

But in natural Japanese, 私は is usually dropped when obvious, so the original sentence is perfectly normal.