Learn Hardcore Japanese: kono suupu no azi ga ii desu.

In Japanese, when you describe something with an い-adjective (like いい), you need a subject marker. 味がいい literally means “the taste is good.” Without が, the sentence would be ungrammatical.

What’s the difference if I replace が with は, as in このスープの味はいいです?

Using は makes 味 the topic, often implying contrast or a general statement.

味がいいです = “The taste is good.” (neutral/fact)
味はいいです = “As for the taste, it’s good.” (perhaps implying other aspects aren’t as good)

Why use 味がいい instead of おいしい?

おいしい is the direct word for “delicious,” while 味がいい says “the flavor/taste is good.”

味がいい can sound slightly more formal or objective and explicitly focuses on the quality of the taste.
おいしい is more casual and common for “tasty” or “delicious.”

What is the role of です at the end, and can I drop it?

です is the polite copula that softens the sentence to polite speech. In casual conversation you can drop it and say このスープの味がいい.

Is there another simple way to say the same thing, like このスープはおいしいです?

Yes. このスープはおいしいです means “This soup is delicious.” It uses は to topic-mark スープ and applies おいしい directly, making it a bit more straightforward.

What kind of adjective is いい, and are there any irregularities?

いい is an い-adjective, but it’s irregular because its dictionary form is よい. In everyday speech you use いい, while よい appears in more formal or set expressions.

Why is スープ written in katakana and not hiragana or kanji?

スープ is a loanword from English “soup,” and Japanese conventionally writes foreign-derived words in katakana.