Learn Danish: Pakken ligger vel i postkassen. - The parcel is in the mailbox, I suppose.

In this sentence, vel shows expectation or a slight assumption. By adding it, the speaker is saying they expect the parcel to be there, translating well to 'I suppose' or 'right?'. Without vel, the sentence would be a plain statement of fact: Pakken ligger i postkassen.

Could I use nok instead of vel here?

Yes, but it changes the nuance. Pakken ligger nok i postkassen means 'The parcel is probably in the mailbox.' You use nok when making a likely guess based on probability, whereas vel is used when you expect something to be true and are almost looking for agreement.

Why does the sentence use ligger (lies) instead of er (is)?

Danish prefers to use positional verbs like ligger (lies) or står (stands) to describe exactly how physical objects are located. Because a parcel is a solid object resting flat inside the mailbox, it ligger there. English speakers often find this tricky because English usually just uses 'is'.

Where does vel have to go in the sentence structure?

vel belongs to a group of short adverbs, just like ikke (not) or ofte (often). In a normal main clause like this one, it must be placed directly after the main verb, which is ligger.