Learn Hardcore Russian: Это нужный ключ.

это is a demonstrative pronoun meaning this (or it in equative clauses). In present‑tense Russian, the copula быть (“to be”) is omitted, so you express “This is…” simply as Это … rather than Это есть ….

Why isn’t there an article like a or the before ключ?

Russian does not have articles. Definiteness or indefiniteness is conveyed by context, word order, or additional words (for example, тот “that” or какой‑то “some”).

Why is ключ in the nominative case here?

In an equative sentence (subject–predicate structure) using это, both parts appear in the nominative case. Here ключ is the predicate noun, so it stays in nominative.

Why do we use the full adjective нужный rather than the short form нужен or the impersonal нужно?

Full‑form adjectives like нужный modify nouns directly (“needed key”) and agree in gender, number, and case. The short form нужен is predicative (“the key is needed”), and нужно is impersonal, used with a dative subject (“мне нужно” – “I need…”).

How does adjective–noun agreement work in нужный ключ?

Adjectives in Russian must match their nouns in gender, number, and case. ключ is masculine singular nominative, so the adjective takes the masculine singular nominative form нужный. With plural ключи you’d say нужные ключи.

Could I place нужный after ключ, i.e. Это ключ нужный?

Although Russian word order is flexible, the neutral, most common order puts adjectives before nouns. Это ключ нужный sounds marked or poetic; in everyday speech you’d say Это нужный ключ.