Teorien er enkel.

Because Norwegian usually puts the definite article on the end of the noun.

• teori = theory / a theory as a dictionary form
• en teori = a theory
• teorien = the theory

So -en here is the definite singular ending.

Teori is a common-gender noun in Bokmål, so its indefinite form is en teori.

That matters because adjectives agree with the noun’s gender and number. Since teori is common gender singular, the adjective in this sentence is enkel.

Er is the present tense of å være (to be).

So in this sentence, er works like English is.

Related forms:

• å være = to be
• er = am / is / are in the present tense, depending on context

Norwegian does not change the verb for different persons the way English does, so:

• jeg er
• du er
• han er
• vi er

all use er.

Because Norwegian adjectives agree with the noun in gender and number.

Here, teorien is:

• singular
• common gender

So the adjective takes the common singular form:

• enkel = common gender singular
• enkelt = neuter singular
• enkle = plural, and also some definite uses before a noun

Compare:

• Teorien er enkel.
• Systemet er enkelt.
• Teoriene er enkle.

Because Norwegian often expresses definiteness by attaching it to the noun instead of using a separate word.

So:

• English: the theory
• Norwegian: teorien

This is one of the biggest structural differences from English.

However, you can also get a separate definite word in phrases with adjectives before the noun:

• den enkle teorien = the simple theory

Here you see both:

• den = definite marker
• enkle = adjective form
• teorien = noun with definite ending

Because enkel is being used predicatively, not directly before the noun.

In Teorien er enkel, the structure is:

• Teorien = subject
• er = verb
• enkel = predicate adjective

This is like English The theory is simple.

If the adjective comes directly before the noun, it is attributive instead:

• en enkel teori = a simple theory

So both are possible, but they are used in different sentence structures.

The basic order here is:

Subject + verb + adjective

So:

• Teorien = subject
• er = verb
• enkel = complement

This looks very similar to English.

But Norwegian is a V2 language, which means the finite verb usually comes in the second position in main clauses. So if you move another element to the front, the verb still stays second:

• Teorien er enkel.
• I dag er teorien enkel.

Notice that er stays in second position.

The noun and adjective would both change:

• Teorien er enkel. = singular
• Teoriene er enkle. = plural

Breakdown:

• teoriene = the theories
• enkle = plural form of the adjective

So the adjective changes from enkel to enkle because the subject is now plural.

Then you would use en teori instead of teorien:

• En teori er enkel.

That means a theory is simple.

So compare:

• en teori = indefinite singular
• teorien = definite singular

The adjective enkel stays the same here because the noun is still common gender singular.