Learn Russian: Нам нужна кровь для девочки после аварии. - We need blood for the girl after the accident.

In Russian, the concept of 'needing' something is flipped. The person who needs something is in the Dative case (нам means 'to us'). The thing that is needed is the grammatical subject, so the word for 'needed' must match its gender. Because кровь (blood) is feminine, we use the feminine form нужна.

How do we know that кровь is a feminine noun?

Nouns ending in a soft sign (-ь) can be either masculine or feminine, and usually need to be memorized. кровь happens to be feminine, which is why it triggers the feminine agreement in нужна.

Why does the word for girl change to девочки here?

The preposition для (for) always requires the Genitive case. To form the Genitive, the dictionary form девочка changes its ending from -а to -и.

Why does the word for accident end in -ии (аварии)?

The preposition после (after) also takes the Genitive case. The dictionary form is авария (accident). For feminine nouns that end in -ия, the Genitive case ending is always -ии.