Сегодня я поеду на метро, а не на машине.

Because the sentence talks about one future trip: today I will go.

• еду = I am going / I’m on my way right now, or sometimes a planned trip in the near future
• поеду = I will go / I will set off in the future

So Сегодня я поеду... means that the speaker is talking about what they will do later today, not what they are doing at this moment.

Поеду is the 1st person singular future form of поехать.

• infinitive: поехать = to go, to set off (by vehicle)
• я поеду = I will go
• ты поедешь = you will go
• он/она поедет = he/she will go

This verb belongs to the family of motion verbs:

• ехать = to be going by vehicle
• поехать = to set off / go by vehicle

So the prefix по- helps create a perfective verb meaning a single trip or the start of movement.

Both can refer to the future, but they are used differently.

• поеду focuses on the fact of going / setting off / one complete trip
• буду ехать focuses on the process of being in transit

In this sentence, Сегодня я поеду на метро... is the normal, natural choice because the speaker is simply saying what means of transport they will use today.

Compare:

• Сегодня я поеду на метро. = Today I’ll go by metro.
• В пять часов я буду ехать на метро. = At five o’clock I’ll be traveling on the metro.

In Russian, means of transportation are very often expressed with на + prepositional case:

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

So на is the normal preposition here.

English says by metro / by car, but Russian usually says literally something closer to on the metro / in the car, depending on the vehicle.

Because with most transport, Russian normally uses на + prepositional:

• на машине
• на автобусе
• на поезде

Even though English uses by, Russian does not usually use the instrumental here.

So:

• Я поеду на машине. = I’ll go by car.

Using машиной in this sentence would sound unnatural for ordinary means of transport.

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

So:

• nominative: метро
• after на: still метро

But машина is a normal declinable noun, so after на it changes to the prepositional form:

• машина → на машине

This is very common with some borrowed words like метро, which do not change endings.

А не is used for contrast: but not / rather than.

So:

• Сегодня я поеду на метро, а не на машине. = Today I’ll go by metro, not by car.

The conjunction а sets up a contrast between two alternatives:

• на метро
• на машине

Using а не makes the correction or contrast sound clear and natural.

Because а is a coordinating conjunction, and in Russian it is normally separated by a comma when it links two contrasted parts of a sentence.

Here the sentence has two contrasted parts:

• Сегодня я поеду на метро
• а не на машине

So the comma is required.

Yes, it could be omitted.

Russian often leaves out subject pronouns when they are already clear from the verb ending:

• Сегодня поеду на метро, а не на машине.

This still clearly means I will go, because поеду already shows first person singular.

Why include я then?

• for emphasis
• for clarity
• because the speaker wants a slightly fuller or more neutral sentence

Both are natural.

Yes. Russian word order is flexible, although the emphasis changes.

Original:

• Сегодня я поеду на метро, а не на машине.

Other possible versions:

• Я сегодня поеду на метро, а не на машине.
• На метро я сегодня поеду, а не на машине.

The original sounds neutral and natural. Putting сегодня first highlights today. Putting на метро first would emphasize the means of transport more strongly.

Because сегодня is an adverb meaning today. Russian time adverbs often appear without prepositions:

• сегодня = today
• завтра = tomorrow
• вчера = yesterday

So Сегодня я поеду... is just the normal way to say Today I will go...

Sometimes, yes, but the meaning or style changes.

• Сегодня я еду на метро, а не на машине.

This can mean:

• Today I’m going by metro, not by car
• or I’m taking the metro today as a present tense used for a planned near-future action

That is possible in Russian. However:

• Сегодня я поеду на метро... sounds more clearly future and more like a single decision or plan for later today.

So both can be possible, but поеду is the clearer future form here.

Yes. ехать / поехать are used for going by some form of transport or while being carried/driven.

Examples:

• ехать на метро = to go by metro
• ехать на машине = to go by car
• ехать на автобусе = to go by bus

If you are going on foot, Russian uses a different verb:

• идти / пойти

So:

• Я поеду на метро. = I’ll go by metro.
• Я пойду пешком. = I’ll go on foot.