Learn Hardcore Russian: Когда началась гроза, гром гремел прямо над портом. - When the thunderstorm started, thunder rumbled right above the port.

Yes, you can switch the order: Гром гремел прямо над портом, когда началась гроза is grammatically correct.
The basic meaning is the same: thunder was rumbling right above the port at the moment the storm began.
The version that starts with Когда началась гроза slightly emphasizes the beginning of the storm, while the reversed order slightly emphasizes the thunder. The difference is more about stylistic focus than about objective meaning.

Why is it Когда началась гроза and not Когда начиналась гроза?

Началась is perfective past (from начаться), and it presents the beginning of the storm as a completed event, a clear starting point.
Начиналась is imperfective past (from начинаться) and would focus more on the process of beginning, like “was beginning” or “was just starting.”
In this sentence we want a definite “start point” in time, so началась is the natural choice.

Why does началась end with -ась? What does that form mean?

The verb is начаться (to begin, start – for something starting by itself), and it is reflexive, which is shown by -ся / -сь.
Началась is the past tense, feminine, singular form: the subject гроза is feminine, so the verb agrees with it.
Masculine would be начался (for a masculine noun), neuter началось, plural начались.

Could we say Когда гроза началась instead of Когда началась гроза? Is there any difference?

Yes, Когда гроза началась, гром гремел прямо над портом is also correct.
Both word orders are normal and very common.
Placing гроза right after когда makes the subject slightly more prominent, but in everyday speech the difference is minimal and both sound natural.

Why is гроза used here instead of шторм or буря?

Гроза means a thunderstorm, with thunder and usually lightning.
Шторм is more about a strong storm, especially at sea or ocean, often focused on wind and waves.
Буря is a more general “storm” or “tempest,” and can be used metaphorically for emotional or political storms.
Because we mention гром (thunder), гроза is the most specific and natural choice here.

Why do we say гром гремел? Isn’t гром already “thunder”? Why repeat the idea with a verb?

In Russian, you normally need a verb in a full sentence; just saying гром над портом would be like saying “thunder above the port” with no verb.
Гром гремел literally means “thunder was rumbling,” combining the noun (гром) and a verb describing its sound (гремел).
This is similar to phrases like дождь шёл (“rain was falling”) or ветер дул (“the wind was blowing”) in Russian.

What is the nuance of гремел? How is it different from грянул or прогремел?

Гремел (imperfective) describes a continuous or repeated action: thunder was rumbling over a period of time.
Грянул or прогремел (perfective) would suggest a more sudden, single loud crash of thunder.
So the sentence with гремел paints a picture of ongoing rumbling while the storm starts.

What does прямо add here? Could we leave it out?

Прямо над портом means “directly/right above the port,” emphasizing closeness and exact position.
Without it, над портом simply means “above the port,” which is correct but less vivid.
You can leave прямо out; the sentence will still be grammatical, just a bit less expressive.

Why is it над портом and not над порт or над портом with some other ending? What case is this?

The preposition над (“over, above”) usually takes the instrumental case.
Порт (nominative) becomes портом in the instrumental singular.
So над портом literally means “over/above the port” and follows the standard preposition + case rule: над + instrumental.

Is портом always instrumental, or can it mean something else?

The base form is порт (nominative). Портом is the instrumental singular; it can appear:

after prepositions like над портом, перед портом, с портом (with “with”),
or as a free instrumental like он стал портом (“it became a port”).
In this sentence, the only reason for портом is the preposition над, which requires instrumental.

Can we say Когда началась гроза, прямо над портом гремел гром with a different word order in the second part?

Yes, Когда началась гроза, прямо над портом гремел гром is correct and sounds natural.
The word order in Russian is relatively flexible, and moving прямо над портом to the beginning of the main clause puts extra emphasis on the location.
The core grammatical roles don’t change: гром is still the subject, гремел is the verb.

Why is there a comma after гроза?

Russian uses a comma to separate the subordinate clause from the main clause.
Когда началась гроза is a subordinate clause introduced by когда (“when”), and гром гремел прямо над портом is the main clause.
In a complex sentence like this, a comma between them is obligatory in standard Russian punctuation.

Could we use как instead of когда, as in Как началась гроза, гром гремел прямо над портом?

Using как in this exact position would sound odd and not standard in this meaning.
Как can sometimes introduce a time-related clause, but it often adds nuances like “as soon as” or can sound colloquial, and here it doesn’t fit well.
Когда началась гроза is the natural, neutral way to say “When the storm began” in standard Russian.

How do you pronounce the stress in гроза, гремел, and портом?

гроза́ – stress on the last syllable: gra-ZA
греме́л – stress on the second syllable: gre-MEL
порто́м – stress on the second syllable: por-TOM
Correct stress is important in Russian; misplaced stress can make words harder to understand.

Is гром always masculine? How do we know that гремел should be masculine?

Yes, гром is a masculine noun.
In the past tense, Russian verbs agree in gender and number with the subject.
So for masculine singular гром, the past-tense masculine form гремел is required (feminine would be гремела, neuter гремело, plural гремели).