Tørk flekken samtidig som du sjekker skrammen på bilderammen.

Because Tørk is an imperative form, used for commands.

In Norwegian, commands usually use just the verb:

• Tørk! = Wipe!
• Sjekk! = Check!
• Kom! = Come!

So Tørk flekken means Wipe the stain/spot. The subject du is normally left out in commands.

Because Norwegian usually puts the definite article at the end of the noun instead of using a separate word like English the.

Examples:

• en flekk = a stain / a spot
• flekken = the stain / the spot

The same pattern appears elsewhere in the sentence:

• en skramme → skrammen
• en ramme → rammen
• en bilderamme → bilderammen

In Bokmål, many nouns that can be feminine also have a common-gender form. So both of these can be possible:

• en skramme → skrammen
• ei skramme → skramma

This sentence uses skrammen, which is very normal in Bokmål. Learners should be aware that both patterns may appear.

Samtidig som means while or at the same time as.

Yes, mens could also work in many contexts:

• Tørk flekken mens du sjekker skrammen på bilderammen.

The difference is mainly nuance:

• samtidig som sounds a bit more explicit about the two actions happening at the same time
• mens is shorter and very common in everyday speech

Because samtidig som introduces a subordinate clause.

In subordinate clauses, Norwegian normally keeps the order subject + verb:

• ... som du sjekker ...

So:

• du sjekker = correct here

A form like sjekker du would sound like question word order, not the normal order after samtidig som.

The command is only in the main clause:

• Tørk flekken

The part after samtidig som is not another command. It simply describes the action happening at the same time:

• du sjekker skrammen på bilderammen

So Norwegian uses normal present tense there, just like English does in while you check.

The verb tørke can mean both dry and wipe, depending on context.

In this sentence, with flekken, the natural meaning is wipe or wipe off:

• Tørk flekken = Wipe the stain/spot

If the context were about making something become dry, then dry would be more likely.

Here på means on.

It tells you where the scratch is:

• skrammen på bilderammen = the scratch on the picture frame

So på bilderammen belongs with skrammen, not with flekken.

It is a compound noun:

• bilde = picture
• ramme = frame
• bilderamme = picture frame
• bilderammen = the picture frame

The r is just the first letter of ramme, not a plural marker. So this is not the plural word bilder followed by something else.

Yes, but it changes the structure a little.

• Tørk flekken samtidig som du sjekker skrammen ...
  = one instruction with a simultaneous action

• Tørk flekken og sjekk skrammen ...
  = two coordinated commands

Both are possible, but samtidig som emphasizes that the actions happen at the same time.