Learn Hardcore Japanese: doyou no yoru ni tomodati to issyo ni nihongo de nagai kaiwa wo simasita. - On Saturday night I had a long conversation in Japanese with a friend.

It’s not possessive in the same emotional sense as “my book,” but grammatically it’s similar to English “Saturday’s night” or “Saturday night.” Japanese uses の very broadly to link nouns in this way.

What’s the difference between 土曜 and 土曜日?

Both mean Saturday.

土曜日 (どようび) – full form, literally “Saturday day”
土曜 (どよう) – shortened form

Nuance:

Both are correct in normal conversation.
土曜 feels a bit more clipped/colloquial or like a schedule label (like writing “Sat” instead of “Saturday”).
土曜日 is a bit more formal/neutral or textbook-style.

In this sentence, 土曜の夜 and 土曜日の夜 mean the same thing: Saturday night.

Why is there a に after 夜? Could I just say 土曜の夜 without に?

に after a time expression marks the point in time when something happens.

土曜の夜に = “on Saturday night”

About omitting に:

With very general times (e.g. 明日 “tomorrow”, 来週 “next week”, 毎日 “every day”), に is often dropped:
- 明日行きます – I’ll go tomorrow.
With more specific points in time (clock times, dates), に tends to be used:
- ３時に行きます – I’ll go at three o’clock.

土曜の夜 is somewhere in between. You can say either:

土曜の夜、友達と…しました。
土曜の夜に、友達と…しました。

Both are natural. に makes the time-marking a bit more explicit and slightly more careful/formal.

What exactly does 友達 (ともだち) mean here? Is it “friend” or “friends”?

Japanese nouns usually don’t show singular/plural by form.

友達 can mean:
- “a friend”
- “the friend”
- “friends”

In this sentence, context would decide whether it’s one friend or several. If you need to specify:

友達一人 – one friend
友達二人 – two friends
友達たち – friends (possible, but 〜たち with 友達 is less common than just relying on context)

So 友達と一緒に can be “together with a friend” or “together with (my) friends.”

I’ve seen 友達 and 友だち. Is there a difference?

Meaning: no difference. Both are read ともだち and mean “friend(s).”

友達 – uses kanji for both parts; very common in print.
友だち – mixes kanji and hiragana; also correct and sometimes preferred for readability, especially for learners or children.

In everyday adult writing, 友達 is more common, but both are fine.

Why do we need both と and 一緒に in 友達と一緒に? Don’t they both mean “together with”?

They play different roles:

と after a noun marks a companion/partner:
- 友達と = “with (my) friend(s)”
一緒に is an adverb meaning “together”:
- 一緒に = together

Combined:

友達と一緒に = “together with (my) friend(s)”

You have options:

友達と一緒に日本語で長い会話をしました。
– Very clear: “I had a long conversation in Japanese together with my friend(s).”
友達と日本語で長い会話をしました。
– Natural; still understood as “with my friend(s).” 一緒に is implied.
友達と一緒に without と (i.e. 友達一緒に) is wrong.
You need と after the companion noun; 一緒に on its own can appear, but not glued directly to 友達 without と.

So と = the companion marker, 一緒に = the “together” adverb; they often go together but don’t mean exactly the same thing.

What does で mean in 日本語で? Why not use に or something else?

Here, で marks the means or medium by which an action is done.

日本語で = “in Japanese / using Japanese (language)”

This use of で is common:

日本語で話す – speak in Japanese
英語で書く – write in English
車で行く – go by car
箸で食べる – eat with chopsticks

Compare:

に – often marks destination, target, or time point: 学校に行く “go to school,” ３時に “at 3:00.”
を – marks direct object: 本を読む “read a book.”

So 日本語に or 日本語を would be wrong here; で is exactly the particle for “by/with/in (as a means).”

Why is it 長い会話をしました instead of 長く会話しました?

Both are grammatically possible, but they’re different structures.

長い会話をしました。
- 長い (long) directly modifies the noun 会話 (conversation).
- Literally: “(I) did a long conversation.”
- Pattern: [adjective] + [noun] + をする
長く会話しました。
- 長く is the adverbial form of 長い, modifying the verb 会話しました.
- Literally: “(I) conversed for a long time.”
- Pattern: [adverb] + [verb]

Nuance:

長い会話をしました focuses on the conversation itself as “a long conversation.”
長く会話しました focuses a bit more on the duration of the action “we conversed for a long time.”

In everyday speech, 長い会話をしました sounds very natural and is probably a bit more common.

What’s the difference between 会話 and 話 / 話し?

All are related to “talking,” but they differ:

会話 (かいわ) – “conversation,” usually two-way exchange, often a bit more neutral/formal or textbooky.
- 日本語の会話 – Japanese conversation
- 友達と会話する – to have a conversation with a friend
話 (はなし / はなし) – “talk, story, what someone says.”
- 面白い話 – an interesting story
- その話を聞きました – I heard that story / what you said
Verb 話す (はなす) – “to speak, to talk.”

In this sentence, 会話 is good because it suggests a back-and-forth conversation, not just one person speaking at the other.

Where is the subject “I” in this sentence? How do we know it means “I had a long conversation”?

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

しました is the polite past of する (“to do”).
In a typical context where you’re talking about your own activities, the default understood subject is “I / we.”

So:

土曜の夜に友達と一緒に日本語で長い会話をしました。
→ “[I] had a long conversation in Japanese with my friend(s) on Saturday night.”

If you want to explicitly say “I”:

私は土曜の夜に友達と一緒に日本語で長い会話をしました。
– “As for me, I had a long conversation…”

But usually 私は is dropped if it’s obvious you’re speaking about yourself.

Is this word order fixed, or can I move parts like 日本語で or 土曜の夜に around?

Japanese word order is relatively flexible, as long as:

The verb comes at the end, and
Particles stay attached to the words they mark.

Your sentence:

土曜の夜に　友達と一緒に　日本語で　長い会話を　しました。

Common alternative orders (all natural):

友達と一緒に土曜の夜に日本語で長い会話をしました。
日本語で土曜の夜に友達と一緒に長い会話をしました。
土曜の夜に日本語で友達と一緒に長い会話をしました。

Typical neutral order is often:

Time (土曜の夜に)
Participant / companion (友達と一緒に)
Means / language (日本語で)
Object (長い会話を)
Verb (しました)

So you have flexibility, but the original ordering already feels very natural.