Sideleiet holder luftveiene åpne.

Because sideleie is a neuter noun in Norwegian.

Its basic forms are:

• et sideleie = a recovery position / a side-lying position
• sideleiet = the recovery position / the side-lying position

So the -et ending marks definite singular for a neuter noun.

Norwegian usually puts the at the end of the noun instead of using a separate word before it.

So:

• sideleiet = the recovery position
• luftveiene = the airways

This is very normal in Norwegian. English says the airways; Norwegian often says luftveiene.

A useful extra point: if there is an adjective before the noun, Norwegian often uses double definiteness, for example:

• det stabile sideleiet = the stable recovery position

Holder is the present tense of å holde.

In this sentence, it means keeps, not just holds in the physical sense.

So:

• holder luftveiene åpne = keeps the airways open

This is a very common use of å holde:

• holde døren åpen = keep the door open
• holde vinduene lukket = keep the windows closed

No. Here åpne is an adjective, not a verb.

In this sentence:

• holder = the verb
• luftveiene = the object
• åpne = describes the state of the object

So the pattern is:

• holde + object + adjective

Meaning:

• keep + something + open/closed/warm/etc.

Compare:

• Sideleiet holder luftveiene åpne = The recovery position keeps the airways open.
• Sideleiet åpner luftveiene = The recovery position opens the airways.

That second sentence uses åpner, which is the verb opens.

Because åpne agrees with luftveiene, which is plural.

The adjective åpen changes form:

• åpen = singular common gender
• åpent = singular neuter
• åpne = plural, and also many definite uses

So:

• en dør er åpen = a door is open
• et vindu er åpent = a window is open
• luftveiene er åpne = the airways are open

The same agreement happens here after holder:

• holder luftveiene åpne = keeps the airways open

Luftveiene is a compound noun.

It comes from:

• luft = air
• vei = way / passage / route

So luftvei literally means airway.

Its forms are:

• en luftvei = an airway
• luftveier = airways
• luftveiene = the airways

The ending -ene here marks definite plural.

Yes. This is a normal Norwegian main clause:

• Sideleiet = subject
• holder = verb
• luftveiene = object
• åpne = object complement / describing the object

So the structure is:

• Subject + Verb + Object + Adjective

Very natural Norwegian.

Also, Norwegian main clauses follow the V2 rule, which means the finite verb is usually in the second position. If another element came first, the verb would still stay second:

• I førstehjelp holder sideleiet luftveiene åpne.

That is also normal.

Because Norwegian usually writes compound nouns as one word.

So:

• side + leie → sideleie
• luft + vei → luftvei

This is much more common in Norwegian than in English. English often uses two words or a hyphen, but Norwegian normally joins them into one word.

Yes, grammatically you could say:

• Sideleiet holder luftveien åpen

That would mean The recovery position keeps the airway open.

But it changes the meaning from plural to singular:

• luftveien = the airway
• luftveiene = the airways

And the adjective must match:

• luftveien åpen = singular
• luftveiene åpne = plural

In medical or first-aid language, the plural airways is very common, so luftveiene åpne sounds natural.

It is grammatically possible, but it sounds less natural in this kind of general statement.

The original sentence uses definite forms:

• Sideleiet = the recovery position
• luftveiene = the airways

That sounds like a standard, established fact.

If you remove the definite endings, it becomes more general or abstract:

• Sideleie holder luftveier åpne

A Norwegian speaker would usually prefer the original sentence in a first-aid context.