Learn Hardcore Russian: Ранний поезд прибывает в город утром. - The early train arrives in the city in the morning.

Ранний is an adjective meaning early (describing a noun), while рано is an adverb meaning early (describing a verb).
In this sentence we are talking about what kind of train it is – an early train – so Russian uses the adjective: ранний поезд.
If you used рано, it would need to describe the verb: for example, поезд рано прибывает – the train arrives early.
So:

ранний поезд = early train (adjective + noun)
поезд рано прибывает = the train arrives early (noun + adverb + verb)

What form is ранний, and how does it agree with поезд?

Ранний is a masculine, singular, nominative adjective.
It agrees with поезд, which is also masculine, singular, nominative.
In Russian, adjectives must match the noun in:

gender (masculine / feminine / neuter)
number (singular / plural)
case (nominative / accusative / etc.)

So:

masculine nominative: ранний поезд
feminine nominative: ранняя машина (early car)
neuter nominative: раннее утро (early morning)
plural nominative: ранние поезда (early trains)

Why is the verb in the present tense прибывает when we’re talking about a timetable (future arrival)?

Russian often uses the simple present tense for scheduled or regular future events, especially with transport: trains, planes, buses, TV programs, etc.
Ранний поезд прибывает в город утром can mean The early train arrives in the city in the morning in a general, timetable sense (not just this one time).
You can also use the future if you mean a specific future instance:

Ранний поезд прибудет в город утром. – The early train will arrive in the city in the morning (this coming time).

So:

прибывает = present, used for regular/scheduled actions.
прибудет = simple future, usually one specific future event.

What is the difference between прибывает and прибудет?

They are two aspects of the same verb прибыть / прибывать (to arrive):

прибывать – imperfective
- present: прибывает – arrives / is arriving
- past: прибывал – was arriving / used to arrive
прибыть – perfective
- future: прибудет – will arrive (completed in the future)
- past: прибыл – arrived (completed in the past)

In the sentence Ранний поезд прибывает в город утром, the imperfective прибывает focuses on the process or regularity (what usually or always happens).
If you said прибудет, you would emphasize the result of this one future arrival.

Why is it в город, not в городе?

В город (with accusative) expresses motion towards a place – into the city, to the city.
В городе (with prepositional case) expresses location in a place – in the city.

Compare:

Поезд прибывает в город. – The train arrives to the city (direction, destination).
Поезд стоит в городе. – The train is standing in the city (location, no movement).

Here the train is moving and coming to a destination, so Russian uses в + accusative: в город.

How can I tell that город is in the accusative case if it looks the same as nominative?

Masculine inanimate nouns like город have the same form in the nominative and accusative singular:

nominative: город – the city (subject)
accusative: в город – into the city (object of motion)

You know it is accusative here because of the preposition в plus the meaning of motion towards a place.
So the form doesn’t change, but the function (case) is determined by the preposition and context.

Why is it утром and not в утро?

With parts of the day referring to a time when something happens (in the morning, in the evening, etc.), Russian often uses the instrumental case without a preposition:

утром – in the morning
днём – in the daytime / in the afternoon
вечером – in the evening
ночью – at night

So утром literally is by/with morning but functions as a standard time expression: in the morning.
В утро is not used in this sense in modern standard Russian; утром is the correct idiomatic form here.

What case is утром, and what does that case normally mean?

Утром is the instrumental singular of утро (morning).
The instrumental case has several uses, and one common one is for time when something happens, especially with parts of the day.

So:

утро (nominative) – morning
утром (instrumental) – in the morning (when?)

Other examples:

Я работаю вечером. – I work in the evening.
Он уехал ночью. – He left at night.

Is there any way to know if this means the early train or an early train, since there are no articles in Russian?

Russian has no articles like a/an or the.
The phrase ранний поезд is neutral; it can mean an early train or the early train depending on context.
English speakers must supply the article according to what sounds natural in English and what the situation implies.

For example:

On a timetable: Ранний поезд прибывает в город утром. → The early train arrives in the city in the morning.
In a more general statement about any early train: it could be translated as An early train arrives in the city in the morning.

The Russian grammar itself does not mark this difference; context does.

Can I change the word order, like Утром ранний поезд прибывает в город? Does it change the meaning?

Yes, Russian word order is flexible, and Утром ранний поезд прибывает в город is grammatically correct.
The basic meaning (who arrives where and when) stays the same, but the emphasis shifts slightly:

Ранний поезд прибывает в город утром. – neutral; slight focus on the train and the fact it arrives in the morning.
Утром ранний поезд прибывает в город. – starts with in the morning, so the time is emphasized more.

In most neutral contexts, the original order is natural, but both are acceptable.

How is ранний pronounced, and why are there two н’s?

Ранний is pronounced approximately [RAHN-nee], with stress on the first syllable: ра́нний.
The double нн reflects a historical formation of the adjective and produces a long /n/ sound.
The ий ending here is the standard masculine nominative adjective ending (like новый, синий).

So:

spelling: ранний
stress: ра́нний
pronunciation: [RAHN-nee], with a long /n/ and a soft final -ий.