Learn Hardcore Japanese: kyou ha ii hi desu.

は is the topic‐marker particle. It tells us that 今日 (“today”) is the topic of the sentence – “As for today…”. Although written with the hiragana は, when used as a particle it is pronounced wa due to historical sound changes.

Why are there spaces between the words? Japanese usually doesn’t use spaces.

In standard Japanese writing, words are not separated by spaces. Here, spaces have been added for learners to highlight the boundaries between particles (like は), nouns (like 今日, 日), adjectives (いい), and the copula (です).

How do you pronounce 今日 and 日 in this sentence?

今日 is pronounced きょう (kyō). 日 meaning “day” is pronounced ひ (hi) when it follows an adjective, as in いい日 (“good day”).

Why is いい directly modifying 日 without な? Don’t adjectives need な before a noun?

Japanese adjectives split into two groups:

い-adjectives (ending in い), like いい, can directly modify nouns: いい日.
な-adjectives, like 静かな, require な before the noun: 静かな人.

Sometimes I see いい written as 良い or as よい. What’s the difference?

The original or “dictionary form” of the adjective is よい (often written with the kanji 良い). Over time, the spoken form shifted to いい. They mean the same thing, but いい is far more common in modern speech.

Why isn’t there a particle like に after 今日? Don’t time words usually take に?

Time words can take に when specifying a precise point (e.g. ３時に会う, “meet at 3 o’clock”). Here, however, 今日は uses は to mark “today” as the topic of a general statement (“As for today, it’s a good day”).

Can I say 今日がいい日です instead? What’s the difference between は and が here?

Yes, you can say 今日がいい日です, but it changes the nuance:

は presents 今日 as known information or the topic.
が highlights 今日 as the subject among alternatives (“It is today that is a good day,” perhaps contrasting with yesterday or tomorrow).

Is です necessary here? How would I say this casually?

です is the polite copula. In casual speech you can:

Drop it entirely: 今日はいい日
Use the plain copula だ: 今日はいい日だ

How do I say “Today was a good day”?

Change です to its past form でした: 今日はいい日でした.

How can I say “Today is a very good day”?

Add an adverb before いい, for example:

今日はとてもいい日です
今日はすごくいい日です