Questions & Answers about Oljan måste fyllas på nu.
How do we form the passive verb fyllas here?
To make the passive infinitive, you just take the regular infinitive form of the verb, which is fylla (to fill), and add an -s to the end. Because it follows the modal verb måste (must), we use this infinitive form. So fylla becomes fyllas (to be filled).
Why is the word på used instead of translating "up" as upp?
In Swedish, the phrasal verb for "topping up" or "refilling" something (like an engine with oil, or a water bottle) is fylla på. While English uses "up", Swedish uses the particle på (on).
Can I combine the particle and the verb into one word, like påfyllas?
Not as the main verb of this sentence. When using a phrasal verb, the verb and the particle stay separate, even in the passive voice. You keep the regular word order: first the passive verb fyllas, then the stressed particle på. You might see combined forms as nouns, like en påfyllning (a refill), but the verb itself stays split here.
Is oljan the subject or the object of this sentence?
Grammatically, oljan is the subject of the sentence, which is why it comes first. However, because the sentence is in the passive voice, the subject is the thing receiving the action. The oil isn't filling anything itself; it is the thing being topped up.