Learn Hardcore Japanese: watasi ha tomodati wo taisetu ni simasu.

友達は大切です。
“Friends are important.” (友達 is the topic/subject; 大切です is the predicate.)
友達が大切です。
“(It’s) friends that are important.” (Focusing/contrasting friends with other things.)

With 大切にする, you almost always mark the thing you value with を:
時間を大切にする – “to value your time”
家族を大切にする – “to cherish your family”

What does 大切にします literally mean? Why not just 大切です?

大切 is a na-adjective meaning “important,” “precious,” “cherished.”
～にする can mean “to make (something) ~ / to treat (something) as ~ / to act ~ toward it.”

So:

大切です → “(It) is important.” (a state)
大切にします → “(I) treat (it) as important / cherish it / value it.” (an action / attitude)

In 友達を大切にします, you’re describing what you do / how you behave toward friends, not just saying “friends are important (as a fact).”

Why is there a に after 大切?

For na-adjectives and nouns, turning them into an “adverb-like” form often uses に:

静か (quiet) → 静かに話す “speak quietly”
安全 (safe) → 安全に運転する “drive safely”

Similarly:

大切 (important/cherished) → 大切にする

Here 大切に modifies the verb する, giving something like:

“to do (things) in an important way toward [object]”
“to treat [object] as important”

So 大切します is wrong; it must be 大切にします.

Why is 私 used here? Can I drop it?

You can absolutely drop 私:

友達を大切にします。

Japanese usually omits the subject when it’s obvious from context. Saying 私:

is useful in textbooks and first-time introductions to show the structure clearly,
can sound a bit more deliberate or contrastive, like “I, for my part, value my friends.”

In natural conversation, if context is clear, people would often just say 友達を大切にしてる or 友達を大切にしてます.

Why doesn’t the sentence say 私の友達 (my friends)? How do we know whose friends?

Japanese often omits possessives like my, your, etc. if they’re clear from context.

友達 can be “my friend(s),” “your friend(s),” “our friend(s)” depending on the situation.
In a self-introduction or when talking about your own way of life, 友達 will naturally be understood as “my friends.”

If you really want to stress that they are your own friends, you can say:

自分の友達を大切にします。 – “I take good care of my own friends.”

But in most cases, 友達を大切にします is enough and feels more natural.

Is 友達 singular or plural here?

Japanese nouns usually do not mark singular vs plural by default.

友達 can mean “a friend,” “friends,” or “my friends” depending on context.
Here, in English, it’s typically translated as plural: “I value my friends.”

If you really need to emphasize plural, you can say:

友達たちを大切にします。

but this feels a bit heavy or childlike; it’s not needed in normal speech.

Does します here mean “I will value my friends” or “I value my friends (in general)”?

The non-past form ～します in Japanese can express:

habit / general statement:
“I (habitually, in general) value my friends.”
future / intention:
“I will value my friends.” / “I’m going to value my friends (from now on).”

Which one it sounds like depends on context. With no context, English speakers usually translate this sentence as a general statement: “I value my friends.”

What’s the difference between 友達を大切にします and 友達が大切です?

They’re close in meaning but different in focus and grammar:

友達を大切にします。
- Structure: [I] (topic omitted) + 友達を (object) + 大切にします (verb phrase)
- Focus: what I do / how I act
- Meaning: “I value/cherish my friends; I treat my friends as important.”
友達が大切です。
- Structure: 友達が (subject) + 大切です (predicate)
- Focus: what is important
- Meaning: “Friends are important.”

Both can describe your values, but:

を大切にする = “to actively take good care of / value (something).”
が大切です = “(something) is important (as a fact).”

Would 友達を大切にしています be more natural than 友達を大切にします?

Both are natural, but there’s a nuance difference:

友達を大切にします。
- non-past simple
- can sound like a principle, a policy, or a general statement about yourself.
- often used in self-introductions, mottos, etc.
友達を大切にしています。
- ～ている form
- emphasizes an ongoing state / way of living:
  “I (am) someone who values my friends (and am doing so now / as a continuing habit).”

In everyday conversation, 大切にしている／してます is extremely common when describing how you actually live your life.

Why do we use する instead of another verb like 思う or 愛する?

大切にする is a very common set phrase, meaning roughly:

“to value,” “to cherish,” “to take good care of,” “to treat as important.”

Comparisons:

友達を大切にする。
Neutral, common, natural; includes both feeling and behavior.
友達を大切に思う。
Literally “to think of friends as important.”
Focuses on your inner feeling rather than outward behavior.
友達を愛する。
“To love friends.”
Grammatically fine, but 愛する is very strong and poetic; for friends it can sound overly dramatic or romantic.

In everyday Japanese, 大切にする is the most natural and versatile choice here.

Is 大切 the same as 大事? Could I say 友達を大事にします?

Yes, you can say:

友達を大事にします。

大切 and 大事 overlap a lot. Differences (often subtle and context-dependent):

大切
- slightly more formal / “textbook-ish”
- often used about things you emotionally value / cherish.
大事
- very common in casual speech
- can mean “important,” but also “serious” (大事な問題 = “a serious/important problem”).

In this particular sentence, both are fine. Many speakers would naturally say 友達を大事にしてる in casual conversation.

What grammar pattern is 大切にする an example of?

It shows a common pattern:

[na-adjective / noun] + に + する

Core ideas of this pattern:

to make something ~
- 部屋をきれいにする。 – “make the room clean.”
to treat something as ~ / decide on ~
- ご飯を少なめにする。 – “make the rice portion small.”
- 色は赤にします。 – “I’ll go with red (for the color).”
to act ~ toward the object (our sentence):
- 友達を大切にする。 – “to treat friends as important / to cherish friends.”

So 大切にする is “to act in a way that treats [object] as 大切.”