Learn Russian: Стая птиц летит сквозь туман.

Лететь is a unidirectional verb of motion. It is used here because the flock is flying in one specific direction (through the fog) right at this moment. If the birds were flying around aimlessly in multiple directions, or if we were talking about a general habit, we would use the multidirectional verb летать (летает).

Since there are multiple birds, why is the verb летит in the singular form?

The grammatical subject of the sentence is the singular noun стая (a flock). The word птиц (of birds) is in the genitive case and simply modifies the subject. In Russian, the verb must agree with the head noun, so it takes the singular form летит to match стая.

Why is the word for birds птиц rather than птицы?

It is in the genitive plural because it means 'of birds' (a flock of birds). For most feminine nouns that end in -а in the nominative singular, like птица, the genitive plural form drops the vowel ending entirely, resulting in a word that ends in a consonant.

What case does the preposition сквозь take, and how does it differ from через?