Learn Icelandic: Haustið byrjar bráðum.

In Icelandic, we usually use the definite article (the -ið at the end of haustið) when talking about seasons in a general sense or as a specific upcoming event. While in English you might just say 'Autumn begins soon', Icelandic prefers 'The autumn begins soon'.

If I wanted to talk about the other seasons, how would I write them in this sentence?

You would use their definite forms just like haustið! You would say Veturinn byrjar bráðum (The winter), Vorið byrjar bráðum (The spring), or Sumarið byrjar bráðum (The summer).

Why does the verb end in -ar (byrjar) here?

Byrjar is the third-person singular form of the verb byrja (to begin). Because haustið acts as an 'it' (third-person singular), we add an -r to the verb stem byrja-, making it byrjar.

The word bráðum ends in -um. Is it a plural noun?

No, bráðum is an adverb meaning 'soon'. You have probably seen the -um ending on plural nouns in earlier lessons, but here it is just a fixed word. Because it is an adverb, it never changes its shape.