Learn Hardcore Japanese: kinou ha tomodati to kenka wo site simaimasita. - Yesterday I ended up getting into a fight with my friend.

昨日は literally means “as for yesterday” or “speaking about yesterday”. It sets 昨日 (yesterday) as the topic of the sentence, and then you say what happened yesterday.

You could also say:

昨日、友達とけんかをしてしまいました。

This is also natural. The difference is subtle:

昨日は…
→ Slightly emphasizes or contrasts yesterday (e.g., compared to today, or to some other time already mentioned).
昨日、…
→ Just gives the time frame, more neutral, without that topic/contrast feeling.

You would not use が here after 昨日, because が marks a grammatical subject, and 昨日 is not the subject; it’s just a time expression and/or topic.

Can the word order change? For example, can I say 友達と昨日けんかをしてしまいました?

Japanese word order is flexible as long as the verb stays at the end and the particles stay attached to their words.

Natural variations include:

昨日は友達とけんかをしてしまいました。 (original)
昨日、友達とけんかをしてしまいました。
友達と昨日けんかをしてしまいました。 (possible)

All of these are understandable and acceptable.

However, some orders feel awkward:

友達とけんかを昨日してしまいました。
→ Grammatically possible, but sounds unnatural and marked; it feels like you’re putting strange focus on 昨日.

General guideline: keep time expressions like 昨日 near the beginning, and keep everything that depends on the verb (friends, objects, etc.) before the verb but in a relatively natural order: [time] [participants] [object] [verb].

Why is と used after 友達? I thought と meant “and”, but here it seems to mean “with”.

と does both jobs in Japanese:

Connecting nouns (A and B):
- リンゴとバナナ = apples and bananas
Marking a companion (do something with someone):
- 友達と行く = go with (a) friend
- 家族と食べる = eat with (my) family

In 友達とけんかをする, と shows that the friend is the person you’re fighting with:

友達とけんかする
= to have a fight with a friend

So here と is the “companion/partner” marker rather than a simple “A and B” list.

How do we know it means “with my friend” and not “with a friend” or “with friends”?

Japanese often omits possessive words like “my”, “your”, etc.

友達 alone is usually interpreted from context as “my friend(s)” when the speaker talks about their own relationships.

So:

友達とけんかをしてしまいました。
→ In most real situations, this is understood as “I (regrettably) fought with my friend.”

If you need to be explicit, you can add a possessive:

私の友達とけんかをしてしまいました。 = I fought with my friend.
その人の友達とけんかをしてしまいました。 = I fought with that person’s friend.

Number (friend vs friends) is also usually determined from context; 友達 can be singular or plural.

Why is there an を after けんか? I thought けんか was a verb meaning “to fight”.

In Japanese, けんか (喧嘩) is originally a noun meaning “a fight, a quarrel”.

To say “to fight” as a verb, Japanese uses:

けんかをする
→ literally: “to do a fight”

Here, する is “to do”, and を marks けんか as the object of する.
In the sentence:

けんかをしてしまいました

this is:

けんかをする → けんかをして (て-form) → けんかをしてしまいました

So を is there because けんか is being treated as a noun that you “do” with する.

Is けんかをする different from けんかする? Do I have to use を?

You’ll see both in real Japanese:

けんかをする
けんかする

In everyday speech and writing, they are basically the same in meaning. For many noun + する expressions, を is optional, especially in casual style.

So you could also say:

昨日は友達とけんかしてしまいました。

instead of:

昨日は友達とけんかをしてしまいました。

Both mean the same thing; leaving out を here does not change the meaning in any important way.

What exactly is して in けんかをしてしまいました?

して is the て-form of する.

Breaking it down:

けんかをする = to have a fight
けんかをして = having a fight / (and) fight / fight and…

The て-form is used to:

connect verbs together
attach an auxiliary verb after the main action

Here, して connects the main action けんかをする to the auxiliary verb しまう (in しまいました).

So structurally:

けんかをしてしまいました
= (I) did a fight
- しまいました (auxiliary meaning “ended up doing / unfortunately did”)

What extra nuance does 〜てしまいました add compared to just 〜ました?

Compare:

友達とけんかしました。
友達とけんかしてしまいました。

Both describe a fight that happened in the past, but:

〜ました
→ simple, neutral past: “I fought with my friend.”
〜てしまいました (from しまう) often adds:
- a sense of regret or unwanted / unfortunate action, or
- the idea that the action is completed / over with.

In this context (a fight), it naturally sounds like:

“I (unfortunately / regrettably) ended up fighting with my friend.”

So 〜てしまいました shows that the speaker feels bad about it or that it was not what they wanted to happen.

There is no “I” in the sentence. How do we know who did the action?

Japanese often omits the subject when it is clear from context. Pronouns like “I”, “you”, “he”, “she” are frequently left out.

In:

昨日は友達とけんかをしてしまいました。

there is no explicit subject, but in a real conversation:

If you are saying this about yourself, the listener will naturally understand it as “I”.
The polite form 〜ました also makes it very likely this is the speaker talking about their own past action.

If you really want to include it, you can say:

私は昨日は友達とけんかをしてしまいました。

But normally 私は is dropped unless you need to contrast or clarify.

How would I say this more casually to a friend?

The original is polite:

昨日は友達とけんかをしてしまいました。

A common plain/casual version would be:

昨日は友達とけんかしちゃった。

Points to notice:

〜してしまった (plain) often becomes 〜しちゃった in speech.
ました → た for plain past.
You can keep or drop は and を similarly:
- 昨日、友達とけんかしちゃった。
- 昨日、友達とけんかしてしまった。

All of these are natural casual versions.

Are there kanji for けんか? Why is it written in hiragana here?

Yes, けんか can be written with kanji as:

喧嘩

However:

The kanji 喧 and 嘩 are relatively complex and not among the easiest/most common everyday kanji.
It’s very common in modern Japanese to write some words in hiragana even when kanji exist, especially if the kanji are difficult or the word is very frequent.

So both are fine in real Japanese:

昨日は友達と喧嘩をしてしまいました。
昨日は友達とけんかをしてしまいました。

In textbooks and for learners, けんか in hiragana is often preferred for readability.