Learn Hardcore Russian: Молния сверкнула над городом ночью. - Lightning flashed over the city at night.

Subject: молния – this is a feminine noun (like most nouns ending in -я)
So the past tense must be: сверкнула (fem.), not сверкнул (masc.) or сверкнуло (neut.) or сверкнули (plural).

Compare:

Гром сверкнул – The thunder flashed (masculine)
Солнце сверкнуло – The sun flashed / sparkled (neuter)

Why is it сверкнула and not сверкала? What’s the difference?

Both come from the same verb idea “to flash / sparkle,” but they differ in aspect:

сверкнула – perfective (one completed flash, a single event)
сверкала – imperfective (ongoing or repeated flashing)

So:

Молния сверкнула над городом ночью.
→ One flash, it happened and was over.
Молния сверкала над городом всю ночь.
→ Lightning was flashing over the city all night (repeated / continuous).

English often uses context (was flashing, flashed) to show this, while Russian heavily uses aspect.

What is the infinitive of сверкнула, and what is the imperfective form?

Сверкнула is the past feminine singular of the perfective verb сверкнуть (to flash once, to give a flash).

The aspect pair is:

Imperfective: сверкать – to flash (in general), to be flashing
Perfective: сверкнуть – to flash once, to give a flash

Examples:

Молния сверкала над городом. – Lightning was flashing / used to flash over the city.
Молния сверкнула над городом. – Lightning flashed over the city (once).

Why is it над городом and not just над город or над город in another case?

The preposition над (“over, above”) in modern standard Russian almost always takes the instrumental case.

Nominative: город
Instrumental: городом

So you must say:

над городом – over the city

Using над город is ungrammatical in modern standard Russian in this meaning.

The pattern is:

над морем – over the sea
над столом – over the table
над городом – over the city

Does над always mean just “over”? Could it also mean “above” in this sentence?

Yes, над covers both “over” and “above” in this kind of context.

Молния сверкнула над городом ночью. can be understood as:

Lightning flashed over the city at night.
Lightning flashed above the city at night.

Russian doesn’t distinguish “over” vs “above” the way English often does; над is the general preposition for something located higher than something else.

Why does ночью mean “at night”? What case is that?

Ночью is the instrumental singular form of ночь (night).

Russian often uses the instrumental case without a preposition to express “at [time of day]”:

утром – in the morning / at morning (from утро)
днём – in the daytime / during the day (from день)
вечером – in the evening (from вечер)
ночью – at night (from ночь)

So ночью literally is “by night / with night,” but idiomatically it just means “at night.”

Could I say в ночь instead of ночью?

В ночь is possible but much less common and sounds more stylistic or poetic, and usually needs more context (for example, в ту ночь – “that night”).

For a simple, neutral “at night” in a sentence like this, ночью is the normal, most natural choice:

Молния сверкнула над городом ночью. – neutral, everyday style
Молния сверкнула над городом в ту ночь. – that specific night (more pointed, narrative).

Can I change the word order? For example: Ночью молния сверкнула над городом?

Yes, Russian word order is flexible, and all of these are grammatically correct:

Молния сверкнула над городом ночью.
Ночью молния сверкнула над городом.
Ночью над городом сверкнула молния.

They all keep the same basic meaning, but the focus shifts slightly:

Starting with Ночью emphasizes the time: It was at night that lightning flashed…
Starting with Молния sounds like a neutral description beginning with the subject.

For a learner, the original word order is perfectly fine and very natural.

Why is there no word for “the” before молния (“the lightning”)?

Russian simply does not have articles like “a / an / the”.

The bare noun молния can mean:

“a lightning bolt”
“the lightning”
just “lightning” in general

Context decides whether you interpret it as “a” or “the” in English. So:

Молния сверкнула над городом ночью.
→ “Lightning flashed over the city at night.”
→ or “A flash of lightning lit up the sky over the city at night.”

Both are acceptable translations, depending on context.

How would I say “Lightning was flashing over the city all night”?

You need the imperfective aspect (ongoing / repeated action) and an expression for “all night”:

Молния сверкала над городом всю ночь.

Breakdown:

Молния – lightning (subject, feminine)
сверкала – imperfective past, feminine singular (was flashing)
над городом – over the city (instrumental after над)
всю ночь – all night (accusative after всю indicating the duration of time)

Where is the stress in each word of Молния сверкнула над городом ночью?

The main stresses are:

мо́лния – МО́л-ни-я (stress on the first syllable)
сверкну́ла – свер-кну́-ла (stress on ну́)
над – one syllable, unstressed in the sentence rhythm but phonetically clear
го́родом – ГО́-ро-дом (stress on the first syllable)
но́чью – НО́-чью (stress on the first syllable)

So you would say something like: МО́лния сверКНУ́ла над ГО́родом НО́чью.