Om tunnelbanan är försenad, tar hon spårvagnen istället.

Om is the standard word for if when you introduce a condition:
Om X, (så) Y = If X, (then) Y.
Here it introduces the conditional clause Om tunnelbanan är försenad.

Because Swedish uses V2 word order in main clauses: the finite verb (here tar) must come in the second position.
When you begin with something other than the subject (here, the whole Om… clause), the verb still has to be second, so the subject moves after the verb:

• Om tunnelbanan är försenad, tar hon… (correct)
• Om tunnelbanan är försenad, hon tar… (incorrect in standard Swedish)

It’s common and recommended, especially in writing, to put a comma after an initial subordinate clause like this. In more informal writing you may sometimes see it without a comma, but the comma helps readability:

• Om tunnelbanan är försenad, tar hon… (very standard)
• Om tunnelbanan är försenad tar hon… (possible, more informal)

Tunnelbana means the metro/subway.
Tunnelbanan is the definite form: the metro.

For many -a nouns (common gender en-words), definiteness is made by adding -n (often resulting in -an):

• en tunnelbana = a metro
• tunnelbanan = the metro

Spårvagn means tram/streetcar.
Spårvagnen is the definite form: the tram.

This noun is also an en-word, and many consonant-ending en-words form the definite with -en:

• en spårvagn = a tram
• spårvagnen = the tram

Swedish commonly expresses “be delayed” with vara + adjective:

• är försenad = is delayed

Försenad is an adjective meaning delayed/late. Swedish often prefers this structure where English might also use an adjective (is delayed) or a passive (has been delayed).

Adjectives agree with grammatical gender/number:

• tunnelbanan is an en-word (common gender), singular → försenad
• If it were a ett-word singular, you’d often see -t: försenat
• If plural: försenade

So the form försenad matches tunnelbanan.

Yes, and the meaning shifts slightly:

• är försenad = it is delayed (state/condition)
• blir försenad = it gets/becomes delayed (change/event)

In many real-life contexts both work, but blir can emphasize that the delay happens (or happens by then), while är focuses on the situation “if it’s delayed”.

Swedish frequently uses the present tense for future situations, especially in conditional clauses and everyday statements:

• Om X händer, gör jag Y. = If X happens, I do Y.

So Om tunnelbanan är försenad, tar hon… is a normal Swedish way to talk about what she will do in that situation.

Yes—ta literally means take, and ta + a means of transport is very common Swedish:

• ta tunnelbanan = take the metro
• ta bussen = take the bus
• ta spårvagnen = take the tram

You could also use åker (“travels/goes”) in other phrasing, but ta is a very natural choice when you mean choosing a transport option.

Istället means instead. It often appears toward the end of the clause, after the object:

• tar hon spårvagnen istället = she takes the tram instead

You can move it for emphasis, but this placement is very typical.

A few key points for an English speaker:

• tunnelbanan: stress is typically on -ba-: tun-nel-BA-nan
• spårvagnen: spår has a long vowel; å is like a rounded “aw” sound (but Swedish-specific).
• försenad: the ö is a front rounded vowel (not in English); stress is usually on the last syllable: för-se-NAD

If you want, tell me whether you’re aiming for Stockholm or Finland-Swedish pronunciation—vowel quality can differ slightly.