Learn Hardcore Japanese: keeki ha amai desu.

は is the topic marker. It tells us that ケーキ (cake) is what we’re talking about. After marking the topic with は, the rest of the sentence describes or comments on it (“is sweet”). Unlike が, which marks the grammatical subject or new information, は simply sets the theme of the sentence.

Why is ケーキ written in Katakana?

ケーキ is a loanword (from English cake), and Japanese writes borrowed words in Katakana. Native Japanese words use Hiragana or Kanji (for example, sweets in general might be お菓子).

What kind of word is 甘い?

甘い is an i-adjective. You can tell because it ends in い in its dictionary form. I-adjectives describe qualities and can directly act as predicates (as in ケーキは甘い) or modify nouns (e.g. 甘いケーキ, “sweet cake”).

Why is です added after 甘い?

です is the polite copula. Adding です after an i-adjective makes the sentence polite without changing the adjective’s form. So “甘い” becomes 甘いです in polite speech.

Can you omit です and still understand the sentence?

Yes. In casual or informal contexts you can drop です, giving ケーキは甘い. That’s the plain style (informal) version, whereas ケーキは甘いです is polite.

What changes if you use が instead of は: ケーキが甘いです?

Using が marks ケーキ as the grammatical subject and often implies “it is this cake that is sweet,” highlighting or introducing it as new information. In contrast, ケーキは甘いです states a general fact about cake.

Why isn’t there a particle between 甘い and です?

I-adjectives attach directly to です without any extra particle. The structure is simply [topic] は + [i-adjective] + です.

Why is the particle は pronounced wa and not ha?

Although it’s written with the Hiragana character は, when used as a particle it’s pronounced wa. Inside words (not as a particle) it retains its usual pronunciation ha.