Learn Italian: Il taxi arriva tardi oggi. - The taxi is arriving late today.

You would use the question word quando (when). A natural way to ask this is Quando arriva il taxi? Notice how the subject (il taxi) usually moves after the verb when using these question words.

If I wanted to ask why the taxi is late, how do I say that?

You use perché (why). You could ask Perché il taxi arriva tardi? or Perché arriva tardi il taxi? Remember that perché does double duty in Italian: it means both 'why' and 'because'.

The translation says 'is arriving', but isn't arriva just 'arrives'?

Italian uses the simple present tense (arriva) for both 'arrives' and 'is arriving'. You don't need a separate '-ing' form in Italian to talk about simple actions happening right now!

Can I put the word oggi (today) at the beginning of the sentence?

Yes, you can! Italian word order is quite flexible with time words. Oggi il taxi arriva tardi is a perfectly correct and natural sentence.

How do I make the word taxi plural?