Om tåget är försenat, vill jag gärna ta bussen i stället.

Om means if, so it introduces a condition: if the train is delayed.

You would use När for when, usually when you expect something to happen rather than treating it as a condition.

• Om tåget är försenat... = If the train is delayed...
• När tåget är försenat... = When the train is delayed... or Whenever the train is delayed...

So in this sentence, Om is the natural choice because the speaker is talking about a possible situation.

Tåget is the definite form, meaning the train.

In Swedish, the definite article is usually added to the end of the noun:

• ett tåg = a train
• tåget = the train

So Om tåget är försenat means If the train is delayed, not if a train is delayed.

Because tåget is a neuter noun.

Swedish adjectives often change form depending on the gender and number of the noun:

• en buss är försenad
• ett tåg är försenat
• tågen är försenade

Since tåg is an ett-word, the adjective takes the -t form: försenat.

This is because Swedish is a V2 language: in a main clause, the finite verb usually comes in the second position.

The sentence begins with the subordinate clause:

• Om tåget är försenat

After that, the main clause starts, and the verb must come before the subject:

• vill jag gärna ta bussen i stället

So:

• Om tåget är försenat, vill jag... = correct
• Om tåget är försenat, jag vill... = incorrect

This kind of inversion is very common in Swedish.

Gärna often means something like gladly, preferably, or would like to depending on the context.

In this sentence, it softens the statement and makes it sound more natural:

• vill jag ta bussen = I want to take the bus
• vill jag gärna ta bussen = I’d quite like to take the bus / I’d prefer to take the bus

So gärna often adds a sense of willingness or preference.

That is normal Swedish word order.

In the main clause:

• vill = finite verb
• jag = subject
• gärna = adverb

So the order is:

• vill jag gärna ta bussen

This is very typical when a modal verb like vill is used.

Compare:

• Jag vill gärna gå
• Nu vill jag gärna gå
• Om det regnar, vill jag gärna gå hem

Yes. Ta bussen is a very natural expression in Swedish, just like take the bus in English.

It means to choose that method of transport.

You can also say:

• åka buss = go by bus / travel by bus

But in this sentence, ta bussen fits especially well because the speaker is choosing the bus as an alternative.

So:

• ta bussen = take the bus
• åka buss = go by bus

Both are common, but ta bussen sounds especially natural here.

I stället means instead.

Here it shows an alternative choice:

• ta bussen i stället = take the bus instead

You will also often see:

• i stället för = instead of

For example:

• Jag tar bussen i stället. = I’ll take the bus instead.
• Jag tar bussen i stället för tåget. = I’ll take the bus instead of the train.

You may see both i stället and istället.

Both occur, but i stället is very common and often preferred in careful writing. As a learner, using i stället is a safe choice.

So in this sentence, i stället is perfectly normal.

The comma separates the introductory if-clause from the main clause.

In Swedish, commas before or after subordinate clauses are often used less strictly than in English, so you may sometimes see the sentence without a comma:

• Om tåget är försenat vill jag gärna ta bussen i stället.

That said, the comma is completely natural here and helps readability.

Yes, that is also grammatical.

It means essentially the same thing, but the emphasis is slightly different:

• Om tåget är försenat, vill jag gärna ta bussen i stället.
  This foregrounds the condition: If the train is delayed...

• Jag vill gärna ta bussen i stället om tåget är försenat.
  This starts with the speaker’s preference and adds the condition afterward.

Both are correct. The version with Om first is often a bit more natural when the condition is important.

Är försenat describes the state: is delayed.

• är försenat = is delayed
• blir försenat = becomes delayed / gets delayed

In your sentence, the speaker is talking about the situation where the train is already delayed, so är försenat is the most natural choice.

Not really. It is better to understand the whole phrase naturally rather than word for word.

• vill jag gärna ta bussen means something like
  I would like to take the bus or I’d prefer to take the bus

A very literal translation can sound strange in English. With Swedish, it is often better to learn common combinations like:

• jag vill gärna... = I’d like to...
• jag kommer gärna = I’d be happy to come
• du får gärna... = please feel free to... / you’re welcome to...

So gärna is one of those words whose exact English translation depends a lot on context.