sakana ha oisii desu.

は and が both can follow nouns but have different functions:

• は indicates the topic and often implies a general statement or contrast.
• が marks the grammatical subject and is used to present new or specific information.
  Using 魚が would emphasize “it is fish that is delicious” (perhaps contrasting with something else), whereas 魚は simply states “as for fish, it’s delicious” in a general sense.

おいしい is an i-adjective (い形容詞). I-adjectives:

• End with い in their dictionary (plain) form.
• Can directly modify nouns (e.g., おいしい魚).
• Can serve as predicates without a copula, though often paired with です for politeness.

です is the polite copula, which adds formality and politeness. In this sentence, おいしいです is the present-polite form of the adjective. In casual speech, you can omit です and simply say:

• 魚はおいしい。

In English, adjectives usually precede the noun (delicious fish). In Japanese, adjectives:

• Precede nouns when modifying them directly (おいしい魚 = “delicious fish”).
• Follow the topic or subject as predicates (魚はおいしいです = “Fish is delicious”) because they act like verbs in that position.

Add the question particle か at the end:

• 魚はおいしいですか。
  In casual speech, you can drop か and use rising intonation:
• 魚はおいしい？

Conjugate the i-adjective to past form and add です:

• 魚はおいしかったです。
  In casual speech, you can drop です:
• 魚はおいしかった。

Japanese does not use articles like English. Definiteness is determined by context. If you want to specify “the fish” or “that fish,” you can add a demonstrative:

• その魚はおいしいです。 (That fish is delicious.)
• この魚はおいしいです。 (This fish is delicious.)