Сегодня я впервые поехал на метро.

Поехал is the past tense of a perfective verb and usually means set off / went by vehicle, with focus on the start of the trip or the fact that it happened as a single completed event.

In this sentence, Сегодня я впервые поехал на метро means something like Today I went by metro for the first time.

Compare:

• ехал = was going / went by vehicle, often describing the trip in progress
• поехал = set off / went by vehicle, a single completed event
• ездил = went / used to go / made a trip there and back, often about experience or repeated trips

So поехал sounds natural here because the speaker is talking about a first-time event that happened today.

На метро means by metro / using the metro as transport.

This is a standard Russian pattern for transport:

• на автобусе = by bus
• на поезде = by train
• на машине = by car
• на метро = by metro

By contrast, в метро usually means in the metro, meaning inside the metro system / in the subway as a place.

So:

• Я поехал на метро = I went by metro
• Я был в метро = I was in the metro

Because метро is an indeclinable noun in Russian. That means its form stays the same in different cases.

After на in the meaning of transport, Russian often uses a case form, but with метро you do not see a visible change because the word does not decline.

Compare:

• на автобусе — the noun changes
• на машине — the noun changes
• на метро — the noun stays the same

So the grammar is still normal; метро just happens to be one of those nouns that keep one form.

Впервые means for the first time and is an adverb.

So in this sentence it modifies the whole action:

• Сегодня я впервые поехал на метро = Today I went by metro for the first time

You could also say:

• Сегодня я первый раз поехал на метро

That is also correct and very common in speech.

The difference is small:

• впервые sounds a bit more compact and sometimes slightly more formal or literary
• первый раз sounds very everyday and conversational

Both are natural.

Because the past tense in Russian agrees with the gender of the speaker when the subject is я.

• я поехал = I went / I’m male
• я поехала = I went / I’m female

So if a woman says this sentence, she would say:

• Сегодня я впервые поехала на метро.

This is a very important feature of Russian past tense verbs.

Yes. Russian often omits subject pronouns when the meaning is clear from context.

So you may hear:

• Сегодня впервые поехал на метро.

That can still mean Today I went by metro for the first time, especially if it is obvious who is speaking.

However, including я is perfectly normal too, especially if the speaker wants to be clear or slightly emphasize I.

Russian word order is flexible, and placing сегодня first is very natural because it sets the time frame right away.

• Сегодня я впервые поехал на метро.
  Neutral and natural: Today, I went by metro for the first time.

You could also say:

• Я сегодня впервые поехал на метро.
• Я впервые сегодня поехал на метро.

These are possible, but they may shift emphasis a little.

In the original sentence, the focus begins with today, which is a common way to organize information in Russian.

Впервые is an adverb.

It does not change for case, gender, or number. It tells you how / in what way the action happened: namely, for the first time.

In this sentence it modifies the verb phrase:

• впервые поехал = went for the first time

Not exactly. Russian often uses different verbs depending on how someone went.

Поехать specifically suggests movement by some vehicle / transport.

So поехал is more like:

• went by transport
• set off
• rode
• sometimes just went, if the context already makes the transport clear

That fits well with на метро, because the sentence is specifically about going by metro.

If someone went on foot, Russian would use a different verb:

• пошёл = went on foot / set off walking

That sentence is possible, but it has a different feel.

• Сегодня я впервые поехал на метро focuses on the event as a single action: today I took the metro for the first time
• Сегодня я впервые ездил на метро sounds more like today I had the experience of riding the metro for the first time

Also, ездил often suggests a completed trip, sometimes with an implication of going somewhere and coming back, or simply the experience of traveling by that means.

For a simple first-time event happening today, поехал is usually the more straightforward choice.

Usually no, not if you mean by metro.

• на метро = by metro
• в метро = in the metro / into the metro

So:

• Сегодня я впервые поехал на метро = Today I went by metro for the first time
• Сегодня я впервые поехал в метро would sound odd unless you mean something like I went into the metro for the first time, which is a different idea

For transport, use на метро.

Yes, that is one nuance of поехал. The verb often highlights the beginning of the movement: set off / started going.

But in many everyday sentences, especially with a complete time frame like сегодня and an experience marker like впервые, English naturally translates it simply as went.

So grammatically, поехал contains the idea of setting off, but in normal usage here the overall meaning is just:

Today I went by metro for the first time.

Yes, very natural.

A native speaker could easily say:

• Сегодня я впервые поехал на метро.

Some equally natural alternatives are:

• Сегодня я первый раз поехал на метро.
• Я сегодня впервые поехал на метро.
• Сегодня впервые поехал на метро. (if the subject is clear)

All of these are normal; the original sentence is completely idiomatic.