Learn Hardcore Russian: Это важная деталь.

In Russian, the present‐tense copula быть (“to be”) is normally omitted. So Это важная деталь literally means This is an important detail. There is no need to include an equivalent of is in simple present constructions.

What role does Это play in this sentence?

Это is a demonstrative pronoun meaning this (or this is when introducing something). It identifies the noun phrase that follows. Grammatically it’s indeclinable and stays это regardless of gender or number.

What case are важная and деталь in?

They are both in the nominative case, because after Это (acting as a zero‐copula) the noun phrase remains in nominative to express identity or classification.

Why is важная in its feminine form?

Adjectives in Russian agree with the gender, number, and case of the nouns they modify. Since деталь is a feminine noun in nominative singular, важный takes the feminine ending -ая, becoming важная.

How do I know деталь is feminine?

Nouns ending in a soft sign (ь) can be masculine or feminine. деталь is feminine; you can tell by its agreement with feminine adjectives (e.g. важная деталь) and by its declension pattern (genitive singular детали, plural детали).

How is деталь declined in singular and plural?

деталь is a third‐declension (feminine, soft sign) noun.
Singular:
• Nom: деталь
• Gen: детали
• Dat: детали
• Acc: деталь
• Inst: деталью
• Prep: детали

Plural:
• Nom: детали
• Gen: деталей
• Dat: деталям
• Acc: детали
• Inst: деталями
• Prep: деталях

Where is the stress in Это важная деталь?

The stress pattern is:
• Э́-то (É-ta)
• Ва́ж-на-я (VÁZH-na-ya)
• де-та́ль (de-TÁLʲ)