Learn German: Bitte zurückbleiben, weil der Zug abfährt! - Please stand back because the train is departing!

Using the infinitive is a common way to give an impersonal command in German. It doesn't address anyone in particular (no du or Sie), making it perfect for public announcements, signs, or general instructions, such as those heard on a train platform.

Could we use a past participle here instead, like Zurückgeblieben!?

While past participles can sometimes be used for impersonal commands (like Aufgepasst! for 'Pay attention!'), they usually sound much harsher, more abrupt, and highly urgent. For a standard public announcement, especially when combined with Bitte, the infinitive zurückbleiben is the polite and natural choice.

Why is abfährt written as one word at the very end of the sentence?

abfahren is a separable verb (ab + fahren). Normally in a main clause, the prefix ab detaches and goes to the end. However, weil introduces a subordinate clause, which forces the conjugated verb to the very end. When the conjugated stem (fährt) meets its prefix (ab) at the end of the clause, they join back together into one word: abfährt.

How would the word order change if I used denn instead of weil?