Kilden er pålitelig.

Because kilden is the definite singular form of kilde.

• en kilde = a source
• kilden = the source

In Norwegian, the idea of the is often added as an ending on the noun rather than written as a separate word.

Yes, in Bokmål you may also see kilda.

That is because kilde can be treated as a feminine noun:

• en kilde / kilden
• ei kilde / kilda

Both are acceptable in Bokmål. Kilden is very common and often feels a bit more neutral or formal.

Er is the present tense of å være = to be.

So:

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

In this sentence, kilden er pålitelig literally has the structure the source is reliable.

Because pålitelig is a predicate adjective here. It describes the subject kilden after the verb er.

Compare:

• Kilden er pålitelig. = The source is reliable.
• en pålitelig kilde = a reliable source

So the same adjective can appear:

• after the verb: er pålitelig
• before the noun: pålitelig kilde

Sometimes, yes, but not in this sentence.

Here are some useful forms:

• en pålitelig kilde = a reliable source
• et pålitelig vitne = a reliable witness
• pålitelige kilder = reliable sources

So:

• singular common gender: pålitelig
• singular neuter: pålitelig
• plural: pålitelige

A useful detail: adjectives ending in -ig often do not add -t in the neuter singular.

It can cover both ideas, depending on context.

With kilde, pålitelig usually means:

• reliable
• trustworthy
• dependable as a source of information

So it suggests that the source can be trusted and gives solid information.

You would say:

• en pålitelig kilde

That is the indefinite version.

Compare:

• en pålitelig kilde = a reliable source
• kilden er pålitelig = the source is reliable
• den pålitelige kilden = the reliable source

You move the verb to the front:

• Er kilden pålitelig? = Is the source reliable?

This is very common in Norwegian yes/no questions: the verb comes before the subject.

Add ikke after the verb:

• Kilden er ikke pålitelig. = The source is not reliable.

That placement is very typical in Norwegian main clauses:

• subject + verb + ikke
  • rest

Yes, in this simple sentence it is very similar:

• Kilden = subject
• er = verb
• pålitelig = complement/adjective

So the structure is:

• Subject + verb + adjective

But Norwegian has an important rule called V2: in main clauses, the finite verb is usually in the second position.

For example:

• Kilden er pålitelig.
• Vanligvis er kilden pålitelig.

Even when another word comes first, er still stays in second position.