Some grammar rules require a decision, but this one doesn't: a fixed set of German prepositions always governs the Akkusativ, regardless of context. You see the preposition, and the case is already settled.
The list
Six prepositions make up this group, and all six behave the same way: whatever noun follows them lands in the Akkusativ.
| Preposition | Meaning | Example |
|---|---|---|
| durch | through | Ich gehe durchden Park |
| gegen | against / around (time) | Ich fahre gegenden Baum |
| für | for | Das istein Buch füreinen Freund |
| ohne | without | Ich gehe ohneden Schlüssel |
| um | around / at (time) | Wir sitzen umden Tisch |
| bis | until / up to | Ich arbeite bisnächsten Montag |
Where it shows up
The Akkusativ forms you'll see most often after these prepositions → den and einen for masculine nouns, die and eine for feminine, das and ein for neuter. Feminine and neuter look the same as the Nominativ, so the contrast really only shows up in the masculine.
The same pattern carries over to personal pronouns → mich, dich, ihn, sie, uns, euch. fürmich, ohnedich, gegenihn → the same Akkusativ forms turn up every time.
The masculine is the tell
For most genders the Akkusativ looks identical to the Nominativ, which is exactly why the masculine matters: it's the one place the case actually becomes visible.
- Der Park ist schön. Nominativ → subject
- Ich gehe durchden Park. Akkusativ after fixed preposition
den instead of der → that single letter is the tell. Once you've recognized the preposition, you already know that swap is coming.
The pair to know
Fixed Akkusativ prepositions pair naturally with the fixed Dativ list, and together the two cover every preposition where the case never has to be worked out. What's left after those two lists are the two-way prepositions like in, an, auf, which switch between Akkusativ and Dativ depending on meaning, but that's a separate story. For now, these two fixed lists are all you need, and there's no ambiguity left in either of them.
See durch, gegen, für, ohne, um, or bis → the case is already decided. Whatever follows lands in the Akkusativ, and nothing on the list breaks that.