Questions & Answers about Vad biet gör är att slå med vingarna.
Why do we need att before slå? In English, we often drop the "to" and just say "is flap".
In Swedish pseudo-cleft sentences that focus on an action (Vad... gör är...), the action that follows is treated as a full infinitive phrase. Because of this, you must always include the infinitive marker att before the verb.
Why is it slå med vingarna instead of just slå vingarna?
The verb slå generally means "to hit" or "to beat". To express "flapping", Swedish uses the construction slå med (literally "to beat with"). Without med, it would sound like the bee is physically punching its own wings!
The English translation says "its wings", so why does the Swedish use the definite form vingarna instead of a possessive like sina vingar?
In Swedish, when it is completely obvious whose body parts or clothing are being referred to, you usually use the definite form of the noun (like vingarna, "the wings") rather than a possessive pronoun. Using sina vingar wouldn't be grammatically wrong, but the definite form is much more natural and idiomatic here.
Just to double check, why is the definite form of "bee" biet?
The word for "bee" is an ett-word that ends in a vowel (ett bi). To make it definite, you simply add -et, resulting in biet.