Learn Hardcore Japanese: kyou ha tyousi ga ii desu.

は (wa) marks the topic: 今日 (today). It frames what you’re talking about: “As for today…”
が (ga) marks what has the property: 調子 (condition) is the thing that is good.
Overall structure is topic–comment: “As for today, (my) condition is good.”

Whose condition is it? Why is there no “I”?

Japanese often omits pronouns when they’re obvious. Here it usually means “I’m feeling good today.” But context could make it someone/something else:

About a person: 田中さんは今日、調子がいいです。 (Tanaka is feeling good today.)
About a machine: エンジンは今日、調子がいいです。 (The engine is running well today.)

What exactly does 調子 mean? How is it different from 体調 or 気分?

調子: general condition/performance, for people or things. “In good form,” “running well.”
体調: physical health/condition (body). 今日は体調がいい。 “My health is good today.”
気分: mood/feeling (emotional/subjective). 今日は気分がいい。 “I’m in a good mood today.”
Machines/devices: 調子 is standard (パソコンの調子がいい/悪い). 体調/気分 aren’t used for machines.

Why is it 調子がいい and not 調子はいい?

が with いい is the default “X is good” pattern: Xがいい.
は is possible but adds a contrast/topic nuance: 調子はいいですけど、まだ咳が出ます。 “As for my condition, it’s good, but I still cough.”
With a simple statement, 調子がいい is most neutral/natural.

Can I drop particles in casual speech?

Yes, often:

今日、調子がいい。 (drop は)
今日、調子いい。 (drop は and が; common in conversation)
調子いい。 (when “today” is understood) Keep particles in formal writing/speech for clarity.

How do I say it in past, negative, or past negative?

Because いい conjugates as よい:

Past: 今日は調子がよかったです。 (I felt good today.)
Negative: 今日は調子がよくないです。 / …よくありません。 (I’m not feeling good today.)
Past negative: 今日は調子がよくなかったです。 / …よくありませんでした。

Why does いい turn into よかった/よくない?

The dictionary form is よい. いい is the common spoken form, but it conjugates as よい:

よい → よかった (past), よくない (negative), よく (adverb). So you don’t say いくない or いいかった.

Is よい the same as いい?

Meaning is the same. よい feels more formal, careful, or written. いい is the everyday spoken choice. Both are fine with 調子: 調子がいい / 調子がよい.

Can I add ね or よ at the end? What do they do?

今日は調子がいいですね。 invites agreement/softens: “I’m feeling good today, huh.”
今日は調子がいいですよ。 asserts/informs: “I’m telling you, I feel good today.”
今日は調子がいいですよね。 both inform and seek agreement.

What’s a natural casual version? Is いいだ okay?

Casual: 今日は調子がいい。 / 今日は調子いい。 / 調子いいね。
Don’t say いいだ (i-adjectives don’t take だ directly). You can say いいよ, いいね, or いいんだ (with explanatory の/ん).

How do I ask this as a question?

Polite: 今日は調子がいいですか。 (Are you feeling good today?)
Casual: 今日は調子いい？ / 調子どう？ (How’s your condition?) Possible answers: はい、いいです。/ ううん、よくないです。 / まあまあです。

How is 今日 read here? Isn’t 今日は also read こんにちは?

In this sentence, 今日 is read きょう (kyō). The phrase こんにちは uses the same characters 今日は but is a fixed greeting read こんにちは. Don’t read 今日は as こんにちは here. Pronunciation tips:

今日: きょう (kyō)
調子: ちょうし (chōshi)
です: typically pronounced “des” (the う is often devoiced).

Can 調子 be used for things other than people?

Yes. It’s common for devices, tools, or performance:

パソコンの調子がいい/悪い。 (The computer is running well/poorly.)
今日はスイングの調子がいい。 (My swing is in good form today.)

How can I intensify or soften いい?

Intensify: 今日はとても/すごく/かなり調子がいいです。 (very/really/quite)
Soften: 今日はわりと/けっこう調子がいいです。 (fairly/pretty)
So-so: 今日はまあまあです。 or 今日はあまり調子がよくないです。 (not very good)

Can I change the word order?

The neutral order is 今日は調子がいいです. You can move 今日 for contrast/emphasis:

調子がいいです、今日は。 (adds “at least today” feeling) Avoid splitting 調子がいい unnaturally; keep it together.

How do I use this as a modifier, like “a day when I’m in good shape”?

When 調子がいい modifies a noun, change が to の:

調子のいい日 (a day when I’m in good shape)
調子のいいエンジン (a well-running engine)

Does 今日はいいです mean the same thing?

No. 今日はいいです usually means “I’m fine (I’ll pass)” or “Today is fine/okay (as a schedule).” It’s not about your physical condition. 今日は調子がいいです specifically says your condition is good.

What about using も with 今日?

今日は調子がいいです。 (I feel good today.)
今日も調子がいいです。 (I feel good today too/as well.) も implies it was also true on another occasion (e.g., yesterday).