Learn Swedish: Varför skrattar ni? - Why are you plural laughing?

Historically, yes! It translates quite literally to "wherefore" in older English. However, in modern Swedish, varför is simply the standard word for "why". You can just treat it as a single, complete question word.

Why is the verb skrattar placed before the subject ni?

This happens because of the Swedish "V2 rule" (verb-second rule). In questions that start with a question word like varför, the question word takes the first position. The verb (skrattar) must come immediately after it in the second position, which pushes the subject (ni) to the third spot.

Why doesn't the sentence include a word for "are"?

Swedish doesn't use a separate "to be" verb alongside an "-ing" word to show ongoing action like English does. The single present tense verb skrattar translates to both "laugh" and "are laughing".

Why do we use ni here instead of du?

Ni is the plural "you" (like "you guys" or "y'all"), so this sentence is asking a group of people why they are laughing. If you were only asking one person, you would use the singular du: Varför skrattar du?.