Batteriluken er åpen.

Because Norwegian normally writes compound nouns as a single word.
Here, batteri + luke becomes batteriluke.

The last part, luke, is the main noun, and the first part, batteri, tells you what kind of luke it is.

-en is the definite singular ending here. It corresponds to English the.

So:

• en batteriluke = a battery cover / a battery hatch
• batteriluken = the battery cover / the battery hatch

Norwegian often puts the at the end of the noun instead of using a separate word.

Yes, in Bokmål that is also possible if you use feminine forms.

The noun luke can be treated as feminine, so you may see:

• batteriluken
• batteriluka

Both are acceptable in many contexts. Which one is used depends on writing style, dialect, and personal preference.

Because the adjective has to agree with the noun.

Here, batteriluken is singular, and the adjective form used with a singular non-neuter noun is åpen.

Compare:

• døren er åpen = the door is open
• vinduet er åpent = the window is open
• dørene er åpne = the doors are open

So in Batteriluken er åpen, åpen is the correct form.

Because after er, the adjective is a predicate adjective, and predicate adjectives in Norwegian do not take the special definite form.

So you get:

• en luke er åpen
• luken er åpen

But when the adjective comes before the noun, that is different:

• den åpne luken = the open hatch

So åpen is correct after er, even though batteriluken is definite.

Luke usually means something like hatch, flap, cover, or small door, depending on the context.

In this sentence, it refers to the part that opens on a battery compartment or device, so English might use different words depending on the situation, such as:

• battery cover
• battery hatch
• battery compartment door

Yes.

• åpen = open as an adjective, describing a state
• å åpne = to open, the action

So Batteriluken er åpen describes a condition: the battery cover is in an open state.

Er is the present tense of å være, meaning to be.

It links the subject to its state:

• Batteriluken = the battery cover
• er = is
• åpen = open

So the sentence structure is very similar to English: subject + is + adjective.

A rough guide is AW-pen or OH-pen, depending on dialect.

A few useful points:

• å is a separate Norwegian letter
• it has a rounded vowel sound
• the final -en is usually unstressed

The exact pronunciation varies by region, but the important thing is that å is not pronounced like English a in cat.

You move the verb before the subject:

Er batteriluken åpen?

This is the normal way to make a yes/no question in Norwegian.

Compare:

• Batteriluken er åpen. = statement
• Er batteriluken åpen? = question

Yes, but that means something slightly different in structure.

• Batteriluken er åpen = the battery cover is open
• den åpne batteriluken = the open battery cover

The first is a full sentence.
The second is a noun phrase, not a complete sentence by itself.

So both are correct, but they are used differently.