Tonen er rolig.

In Norwegian, the definite article is usually attached to the end of the noun.

• en tone = a tone
• tonen = the tone

So -en is the definite singular ending here.

The base form is tone.

You would normally learn it as:

• en tone = a tone

Then the definite form is:

• tonen = the tone

Tone is a masculine noun in Bokmål, so it takes:

• en in the indefinite singular: en tone
• -en in the definite singular: tonen

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

So:

• er = is / am / are, depending on the subject

Norwegian does not change this verb by person the way English does. For example:

• jeg er = I am
• du er = you are
• tonen er = the tone is

Because rolig is describing the subject through the verb er. This is called a predicate adjective.

Norwegian works like English here:

• Tonen er rolig. = The tone is calm.

Compare that with an adjective placed directly before a noun:

• den rolige tonen = the calm tone

So after er, the adjective usually comes after the verb.

Here, rolig is used after er, so it is a predicate adjective.

In this sentence, rolig is the correct form. Norwegian adjectives can change form in some situations, but rolig stays rolig here.

Useful comparisons:

• en rolig tone = a calm tone
• tonen er rolig = the tone is calm
• rolige toner = calm tones
• den rolige tonen = the calm tone

So the form can change in other contexts, but not in this sentence.

In this simple sentence, yes. It follows the pattern:

• subject + verb + adjective

So:

• Tonen = subject
• er = verb
• rolig = adjective

However, Norwegian is a verb-second language, so if another element comes first, the verb still stays in second position:

• Nå er tonen rolig. = Now the tone is calm.

You usually put ikke after the verb:

• Tonen er ikke rolig. = The tone is not calm.

That placement is very common in Norwegian main clauses.

For a yes/no question, Norwegian usually puts the verb first:

• Er tonen rolig? = Is the tone calm?

So the statement:

• Tonen er rolig.

becomes:

• Er tonen rolig?

You would say:

• den rolige tonen

This is a very common Norwegian pattern called double definiteness:

• den
  • adjective + definite noun

So:

• den rolige tonen = the calm tone

But in your original sentence, the adjective is not placed before the noun, so you say:

• Tonen er rolig.