Il treno è in ritardo, perciò prendo un autobus.

Perciò means “therefore” or “so” and introduces a result or consequence.
– Perché can mean “why?” (in questions) or “because” (to give a cause), but it does not mean “therefore.”
Example:
– Non piove, quindi/non perciò non porto l’ombrello. (It’s not raining, so I’m not bringing an umbrella.)
– Perché piove? (Why is it raining?)
– Piove perché le nuvole sono scure. (It’s raining because the clouds are dark.)

Yes. Quindi, perciò and dunque are largely interchangeable as result‐linkers.
– Perciò is slightly more formal/written.
– Quindi is very common in speech.
You can say: Il treno è in ritardo, quindi prendo un autobus. or …, dunque prendo un autobus.

Italian uses the idiom essere in ritardo (“to be late”).
– In ritardo functions like an adverbial phrase: Il treno è in ritardo.
– While you might see il volo è ritardato in announcements, for vehicles and people in ritardo is far more natural.
Also note that ritardato can have offensive slang uses, so stick with in ritardo.

Italian indefinite articles:
– Use un before masculine singular nouns starting with a vowel or most consonants (un treno, un autobus).
– Use uno only before masculine nouns beginning with s + consonant, z, gn, ps, x, y (uno studente, uno zaino).
Because autobus begins with a vowel, the correct form is un autobus.

In Italian, prendere takes the vehicle directly as an object. You simply say prendere un autobus, prendere il treno, etc.
If you want to emphasize boarding, you could use salire su un autobus, but it’s not needed here.

– perciò: [perˈtʃɔ] – stress on the -ciò (“per-CHÒ”).
– ritardo: [riˈtardo] – stress on the second syllable (“ri-TAR-do”).
– autobus: [ˈautobus] or [awˈtobus], with stress on the au or the to depending on region, but both are accepted.