En liter vann er nok.

Because liter is the noun that decides the article, and liter is a common-gender noun in Bokmål, so it takes en.

The gender of vann does not control the article here. In en liter vann, the structure is basically a liter of water, so the article belongs to liter, not to vann.

Because vann is being used as a mass noun, like water in English.

After a unit of measurement, Norwegian normally uses the substance noun without an article:

• en liter vann
• to kilo ris
• et glass melk

This works much like English a liter of water, except Norwegian usually leaves out the equivalent of of.

In Norwegian, measure expressions are usually made directly, without a word corresponding to of:

• en liter vann
• to kopper kaffe
• tre meter stoff

So en liter vann is the normal way to say it.

If you add av, it usually sounds less natural in a simple measurement phrase. Norwegian strongly prefers the direct construction here.

It can mean either, depending on context.

• en liter vann can mean a liter of water
• it can also mean one liter of water

Usually context tells you which is meant. If the speaker wants to stress the number, they may emphasize it in speech, and sometimes write én to make the meaning one especially clear.

Nok means enough or sufficient.

So En liter vann er nok means that one liter of water is enough.

In this sentence, nok describes the whole amount as being sufficient. It comes after er because it is part of the predicate:

• Det er nok. = It is enough.
• En liter vann er nok. = A liter of water is enough.

This is the normal word order for a simple statement in Norwegian:

subject + verb + complement

So here:

• En liter vann = subject
• er = verb
• nok = complement

Norwegian is a V2 language, which means the finite verb normally comes in the second position in main clauses. In this sentence, the subject comes first, so the verb comes right after it.

Norwegian verbs do not change for person or number the way English verbs do.

So you get:

• jeg er
• du er
• han er
• vi er
• de er

The form er stays the same. That makes Norwegian verbs simpler than English in this respect.

Grammatically, it is treated as a mass noun, not as a countable plural noun.

Like English water, it is something you normally do not count directly. Instead, you count the amount using a measure word:

• en liter vann
• to liter vann
• mye vann

So the thing being counted is the liter, not the vann.

Not if you want the same meaning.

• en liter vann = a liter of water / an amount of water
• en liter med vann = more like a one-liter container with water or a liter-sized thing containing water

So med changes the meaning. For the amount itself, en liter vann is the natural phrase.

Yes, but it means something different.

• En liter vann er nok = a specific amount is enough
• Vann er nok = water is enough / water is sufficient

Without en liter, the sentence no longer gives the quantity. It becomes a more general statement.

Yes, En liter vann er nok is standard Bokmål.

In Nynorsk, you might more often see vatn instead of vann:

• Ein liter vatn er nok.

So the main differences would be:

• en → ein
• vann → vatn

The structure and meaning stay the same.