Learn Swedish: Idag har vi ett viktigt möte. - Today we have an important meeting.

Because it is describing the word möte (meeting), which is an ett-word (ett möte). When you put an adjective right before a singular ett-word, you must add a -t to its base form. The base form is viktig, so it becomes viktigt.

How would the adjective change if we used an en-word instead?

If we swapped ett möte for an en-word like en sak (a thing), the adjective would just use its base form without the extra -t. You would say en viktig sak (an important thing).

Why does the sentence say har vi (have we) instead of vi har (we have)?

This is a great reminder of the Swedish verb-second (V2) rule. Because the sentence starts with the time word idag (today), the verb har must come next to stay in the second position. This forces the subject vi to slide into the third spot.

Can I write idag as two separate words?

Yes, you can! Both idag and i dag are completely correct and mean exactly the same thing. You will see both forms used often, though the one-word version is very popular in modern writing.