Если троллейбус не придёт, я пойду до метро пешком.

This is a very common difference between English and Russian.

In Russian, with a real future condition, the if-clause can use a future form:

Если троллейбус не придёт, я пойду...

Literally, that first verb is future: if the trolleybus doesn’t come / doesn’t arrive.

English normally uses the present after if, but Russian does not have that same restriction. So this is completely normal Russian.

Because the sentence is about one specific future arrival.

• придёт = perfective future, one completed arrival
• приходит = imperfective present, usually habitual or repeated: it comes / it is coming
• будет приходить = repeated future: it will keep coming / will come regularly

Here the speaker means one event: the trolleybus may or may not arrive. That is why придёт is the natural choice.

Пойду is the normal choice here because it means I’ll go / I’ll set off in response to the condition.

Compare:

• пойду = I’ll go, I’ll set off
• иду = I’m going / I go; sometimes present can refer to near future, but it is less natural here
• буду идти = I will be walking; this stresses the process or duration

In this sentence, the speaker is making a simple future decision: if the trolleybus doesn’t come, I’ll go on foot. That is exactly the kind of situation where пойду works well.

Yes. In fact, дойду до метро пешком is often a very natural alternative.

The difference is mostly one of nuance:

• пойду focuses on starting the action: I’ll go / I’ll head off
• дойду focuses on reaching the destination: I’ll walk as far as the metro / I’ll make it to the metro on foot

So:

• Если троллейбус не придёт, я пойду до метро пешком.
  = I’ll go on foot to the metro.
• Если троллейбус не придёт, я дойду до метро пешком.
  = I’ll walk to the metro / I’ll get to the metro on foot.

The second version can sound a bit more precise if the endpoint matters.

Both can be possible, but they are not exactly the same.

• до метро = up to / as far as the metro
• к метро = to / toward the metro

In this sentence, до метро emphasizes the endpoint reached on foot: as far as the metro.

Also, до requires the genitive case. That leads to another useful point: метро is an indeclinable noun, so its form does not change. Even though it is grammatically genitive after до, it still looks the same:

• метро (nominative)
• до метро (genitive in function, same form)

So the grammar is still there, even though the word itself does not visibly change.

Because метро is one of the Russian nouns that are indeclinable.

Normally, до takes the genitive case, and many nouns would change form after it. But метро keeps the same shape in all cases in the singular.

So:

• метро = the metro
• до метро = to/as far as the metro
• у метро = by the metro
• в метро = in the metro

The preposition changes the case function, but the word itself stays метро.

Пешком means on foot, and learners usually just memorize it as an adverb.

Historically, it comes from an instrumental form, and that is why it looks the way it does. In modern Russian, the easiest way to think of it is simply:

• идти пешком = to go on foot
• пойти пешком = to go on foot
• добраться пешком = to get there on foot

So in this sentence, пешком tells you how the speaker will go.

Good question. Russian often uses прийти / придёт for the arrival of transport, especially in timetable-like or public-transport contexts.

So all of these can sound natural depending on context:

• Автобус придёт через пять минут.
• Поезд пришёл вовремя.
• Троллейбус не придёт.

Приедет is also possible for a vehicle, but придёт is very common when talking about scheduled arrival.

So троллейбус не придёт is normal Russian.

Because Если троллейбус не придёт is a subordinate clause, introduced by если.

Russian separates subordinate clauses with commas, so:

• Если троллейбус не придёт, я пойду до метро пешком.

If the clauses were reversed, there would still be a comma:

• Я пойду до метро пешком, если троллейбус не придёт.

So the comma is required by standard Russian punctuation.

No, Russian word order is fairly flexible. The original sentence is neutral and natural:

Если троллейбус не придёт, я пойду до метро пешком.

But you can move things around to change emphasis:

• Я пойду до метро пешком, если троллейбус не придёт.
  Focuses first on what I’ll do.
• Если троллейбус не придёт, пешком пойду до метро.
  Gives extra emphasis to пешком.
• Если троллейбус не придёт, я пешком пойду до метро.
  Also emphasizes going on foot.

So the basic meaning stays similar, but the focus shifts.

Because these are irregular verbs of motion, and their forms have to be learned.

Here are the key dictionary forms:

• прийти → приду, придёшь, придёт
• пойти → пойду, пойдёшь, пойдёт

So:

• придёт comes from прийти
• пойду comes from пойти

A very common learner mistake is to expect something like прийдёт, but the correct form is придёт.

These motion verbs often change stem in ways that are not fully predictable, so it is worth memorizing their main forms separately.