Learn Swedish: Vad löjlig du är! - How ridiculous you are!

In Swedish, exclamations almost always use Vad where English uses 'How'. If you want to say 'How beautiful!' or 'How silly!', you use Vad followed by the adjective or adverb. Using Hur here would sound like you are asking a literal question measuring their exact level of ridiculousness.

What is the word order rule for this kind of exclamation?

The structure is always Vad + adjective/adverb + subject + verb. Notice that the subject (du) comes before the verb (är). If you swapped them to Vad löjlig är du?, it would sound like a confused question rather than an exclamation.

Would löjlig change if I were talking to a group of people instead of just one person?

Yes. Because löjlig is an adjective describing the subject, it must agree with it. If you were talking to multiple people, you would use the plural pronoun ni and add an -a to the adjective: Vad löjliga ni är!

Can I drop the words 'du är' and just say 'Vad löjlig!'?

Yes, in spoken Swedish it is very common to leave out the subject and verb when it is completely obvious who or what you are reacting to. If you are reacting to a general situation instead of a specific person, you would use the neuter form: Vad löjligt! (How ridiculous!).