Learn French: Vous devez creuser avant de semer les graines.

When the subject performing both actions is the same (here, vous is doing both the digging and the sowing), you don't use a conjunction like avant que with the subjunctive. Instead, you use the preposition avant de followed by an infinitive (semer). You would only use the subjunctive if there were a new subject, like "You must dig before I sow" (avant que je sème).

Could I just say avant semer instead of avant de semer?

No, you need the de. The word avant by itself is used before nouns, such as avant l'hiver (before winter). When you want to say "before doing" an action, the rule in French is to always use avant de followed by the infinitive.

Why does creuser attach directly to devez without a preposition, while semer needs de?

The verb devoir (here as devez) acts as a modal verb, which means it is always followed directly by an infinitive without any prepositions like à or de. In contrast, semer is preceded by de only because it belongs to the fixed prepositional phrase avant de.

I see the verb semer (to sow) here. Is there a related word for the seeds themselves?