Learn Hardcore Russian: Вчера был густой туман над рекой. - Yesterday there was thick fog over the river.

In Russian, when you describe that something existed or occurred in the past, you normally use the past tense of быть (to be). Here был (masculine singular) simply states “there was.” Without был, the sentence would sound like a headline or a poetic fragment, e.g. Вчера густой туман над рекой. You could use other verbs like стоял (“stood”), but был is the most neutral way to say “there was” in the past.

Why is the adjective густой used instead of густая, густое, or густые?

Adjectives in Russian agree with the noun they modify in gender, number, and case. Here туман is a masculine, singular noun in the nominative case (because it’s the subject of был). Therefore, the masculine nominative form of the adjective густой is used.

Feminine would be густая (e.g. густая мгла)
Neuter: густое (e.g. густое облако)
Plural: густые (e.g. густые облака)

What case is рекой, and why is над followed by the instrumental case here?

Рекой is the instrumental case of река (river). The preposition над (“above/over”) requires the instrumental case when you indicate location:

над домом – over the house
над рекой – over the river
Using the prepositional case (о реке) or accusative (реку) after над would be ungrammatical in standard Russian.

Could I place вчера somewhere else in the sentence? For example, Был густой туман над рекой вчера?

Yes, Russian word order is relatively flexible. You can move вчера to the beginning, middle, or end to shift emphasis or fit a style:

Вчера был густой туман над рекой. (neutral)
Был вчера густой туман над рекой. (emphasizes “yesterday” as new info)
Был густой туман над рекой вчера. (more colloquial, last mention of time)
All are grammatically correct, but the first option is the most neutral and common in speech and writing.

Is there any nuance between saying над рекой and над рекою?

Both рекой and рекою are instrumental forms of река.

рекой is the modern, most common form.
рекою has an archaic or poetic feel.
In everyday speech or writing, go with над рекой. Use рекою only if you want a literary or old-fashioned tone.

How do I pronounce густой туман над рекой correctly?

Here’s a rough guide:

густой [goos-TOY] – stress on the second syllable
туман [tu-MAHN] – stress on the second syllable
над [nad] – one syllable
рекой [rʲɪ-KOY] – stress on the second syllable
Put it all together with natural Russian intonation:
гу́стой тума́н над реко́й.

Could I replace туман with мгла or another word for fog/mist?

Yes. Russian has several words for fog/mist:

туман (tuman) – neutral “fog”
мгла (mgla) – poetic or literary “mist/darkness”
дымка (dymka) – light haze
For example: Вчера была густая мгла над рекой. Note that мгла is feminine, so you’d say была and густая.