Learn Hardcore Norwegian: Vakten slipper oss inn etter at vi har vist billetter. - The guard lets us in after we have shown tickets.

Vakten is the definite singular of vakt (meaning “guard”). Norwegian adds the suffix -en to a common-gender noun to make it definite (“the guard”).

What does slipper mean here, and which verb does it come from?

Slipper is the present tense (3rd person singular) of å slippe. Combined with the particle inn, å slippe inn means “to let in.” So slipper here means “(he) lets … in.”

Why is oss placed between slipper and inn?

Because å slippe inn is a separable verb. When you insert an object pronoun (here oss = “us”), it goes between the verb stem (slipper) and the particle (inn): slipper oss inn, not slipper inn oss.

What part of speech is inn, and what role does it play?

Here inn is a separable-verb particle (not a preposition). It completes the meaning of å slippe to form å slippe inn, indicating the action of allowing entry (in-ward direction).

Why is the conjunction etter at used here instead of just etter?

Etter alone is a preposition that takes a noun (etter festen = “after the party”). To introduce a subordinate clause (subject + verb), Norwegian uses etter at: etter at vi har vist billetter (“after we have shown tickets”).

What tense and aspect is har vist, and why is it used?

Har vist is the present perfect tense (auxiliary har + past participle vist of å vise). It indicates that showing the tickets is completed before the guard lets you in.

What is the role of billetter here, and could it be singular?

Billetter (tickets) is the direct object of har vist. You could use the singular object billetten (“the ticket”) if you only have one, but billetter is more common when several tickets are shown.

Could you replace etter at with når in this sentence?

Yes. Når (“when”) can introduce a time clause for a condition or future event.
Example: Vakten slipper oss inn når vi har vist billetter. This is equally correct and often used colloquially.