Learn Hardcore Japanese: taisetuna tomodati to kenka wo suru to, kanasii desu. - It is sad when I fight with important friends.

い-adjectives: end in い, e.g. たかい (tall), さむい (cold)
- Before a noun: たかいビル (a tall building)
な-adjectives: take な before a noun, e.g. きれいな, ゆうめいな, 大切な
- Before a noun: 大切な友達 (an important/precious friend)
- On their own with です: 友達は大切です (Friends are important.)

So 大切な is a な-adjective, and な is needed when it directly modifies a noun like 友達.

Why is it 大切な友達 and not 大切友達 or 大切の友達?

You can’t say 大切友達 because 大切 is not a noun here; it’s a な-adjective, and な-adjectives need な before a noun.

You also don’t say 大切の友達 because の is used after nouns, but 大切 is functioning as an adjective, not as a noun, in this sentence.

So the correct patterns are:

Before a noun: 大切な友達
As a predicate: 友達は大切です。

大切の友達 is ungrammatical in standard Japanese.

What does the と after 友達 mean? Is it the same as “and” in English?

The と right after 友達 here means “with”, not “and”.

大切な友達とけんかをする
literally: “to have a fight with an important friend”

This と is the “companion/partner” particle. Common uses:

友達と話す – talk with a friend
先生と会う – meet with a teacher
兄と遊ぶ – play with my older brother

It can also mean “and” in lists (りんごとバナナ = apples and bananas), but here it’s clearly the “with (someone)” meaning.

Could we use に instead of と, like 大切な友達にけんかをする?

No, 大切な友達にけんかをする is not natural.

For “fight with someone”, Japanese almost always uses:

A とけんかする – fight with A

Compare:

友達に怒る – get angry at a friend
友達とけんかする – fight with a friend

So in the sentence, と is the correct particle to mark the other person in a fight.

Why do we say けんかをする? What kind of word is けんか?

けんか is originally a noun meaning “fight / quarrel”.

Japanese often makes verbs by taking a noun and adding する:

勉強する – to study (勉強 = study)
運動する – to exercise (運動 = exercise)
れんしゅうする – to practice (れんしゅう = practice)
けんかする – to fight / quarrel (けんか = fight)

Because けんか is a noun, you can treat it like a normal object with を:

けんかをする – (literally) “to do a fight”

Both けんかする and けんかをする are correct. を is often optional with many noun + する combinations. Using を can feel a bit more explicit or slightly formal, but in everyday speech people say both.

Can we just say けんかする instead of けんかをする in this sentence?

Yes.

You can say either:

大切な友達とけんかをすると、悲しいです。
大切な友達とけんかすると、悲しいです。

Both are natural and mean the same thing. The を in けんかをする is optional here.

What does the と after けんかをする mean? Is it the same と as after 友達?

No, the two と’s are different:

友達と – particle と = “with (friend)”
けんかをすると、悲しいです。 – conjunction と = “when / if”

The second と is a conditional/conjunction. The pattern is:

[plain form] と、[result]

It often means:

“when(ever) X happens, Y happens”
“if X happens, (then) Y”

So:

大切な友達とけんかをすると、悲しいです。
= When/if I fight with an important friend, (I feel) sad.

Important: the first と is part of 大切な友達と, the second と connects the whole clause 大切な友達とけんかをする to 悲しいです.

Why is the verb before this conditional と in dictionary form する, not past した?

The [plain present] + と pattern is used for things that are:

general truths
usual results
habits or things that always (or typically) happen

So けんかをすると、悲しいです。 is talking about a general situation:
“Whenever / when I fight with a close friend, I (always) feel sad.”

If you wanted to talk about one specific event in the past, you would normally use 〜たら or something similar, for example:

大切な友達とけんかをしたら、悲しかったです。
When I fought with an important friend, I was sad.

Here, the sentence uses すると to express a typical emotional reaction, not a one-time past event.

Why is there です after 悲しい? Don’t adjectives already mean “to be ~”?

Yes, い-adjectives like 悲しい already contain the meaning “to be ~”, so:

悲しい。 = “(I am) sad.”

However, in polite Japanese, you usually add です after an adjective:

悲しいです。 – polite
悲しい。 – plain / casual

So:

い-adjective + です is the polite form.
You cannot do this with verbs (e.g. 行くです is wrong in standard Japanese).
But with adjectives and nouns, です works as the polite copula.

In this sentence, 悲しいです makes the whole statement polite.

Whose feeling is 悲しいです referring to? How do we know it’s “I am sad”?

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

Here, the structure is:

大切な友達とけんかをすると、悲しいです。

There is no explicit 私は, but:

The speaker is talking about their own experience (“when I fight with an important friend…”)
Emotions like 悲しい, うれしい, さびしい usually describe the speaker’s feelings, unless another person is clearly marked.

If the speaker wanted to clearly say the friend is sad, they would normally say something like:

大切な友達とけんかをすると、友達が悲しそうです。
When I fight with my important friend, my friend seems sad.

So, by default, 悲しいです here is understood as “I feel sad.”

Why isn’t there a word like 私 (“I”) in this sentence?

Japanese often drops the subject or topic when it’s obvious from context.

Your sentence could be expanded to:

私は大切な友達とけんかをすると、悲しいです。

This is grammatically fine, but 私は is usually omitted if it’s clear that you are talking about yourself. Constantly saying 私は…私は… sounds repetitive and unnatural in Japanese.

So:

Including 私は is allowed, just not necessary.
Leaving it out is more natural in normal conversation or writing when the subject is clear.

What does the comma (、) after と do? Is it required?

The comma 、 marks a pause and helps readability.

In sentences with a conditional or subordinate clause, it’s very common to put a comma between the two parts:

大切な友達とけんかをすると、悲しいです。

It separates:

Condition: 大切な友達とけんかをすると
Result: 悲しいです

The comma is not strictly required grammatically, but it is standard and makes the sentence easier to read, just like a comma in English:
“When I fight with an important friend, I feel sad.”

Why is けんか written in hiragana? Can it be written in kanji?

Yes, けんか can also be written in kanji as 喧嘩.

However:

喧嘩 uses less common, more difficult kanji.
Many everyday words with hard or uncommon kanji are often written in hiragana in modern texts, especially in materials for learners, children, or in casual writing.

So both are correct:

大切な友達とけんかをすると、悲しいです。
大切な友達と喧嘩をすると、悲しいです。

In ordinary writing, けんか in hiragana is very common.

Does 大切な友達 literally mean “important friend”? Does it also mean “close friend” or “best friend”?

Literally, 大切な友達 is “an important / precious friend.”

In natural English, depending on context, it often corresponds to:

a close friend
a dear friend
a friend who is important to me

It does not necessarily mean “best friend” (only one person). For that, Japanese often uses:

親友 (しんゆう) – a (single) close/best friend

Some related expressions:

大事な友達 – very similar to 大切な友達, also “important/dear friend”
仲のいい友達 – a friend I get along with well; a close friend
親友 – (a) best friend / very close friend

In your sentence, 大切な友達 suggests emotionally important, treasured friends.