Porselenet er gammelt.

The ending -et is the definite singular ending for many neuter nouns in Norwegian.

So:

• porselen = porcelain / china
• porselenet = the porcelain / the china

Unlike English, Norwegian usually attaches the to the end of the noun instead of putting it in front.

Because gammelt is the form of the adjective that agrees with a neuter singular noun.

The basic pattern is:

• en gammel stol = an old chair
• ei gammel bok = an old book
• et gammelt hus = an old house

Since porselen(et) is neuter, the adjective becomes gammelt.

In Porselenet er gammelt, the adjective is still agreeing with the noun, even though it comes after er.

These mean different things grammatically.

• det gamle porselenet = the old porcelain
  
  • here the adjective is placed before the noun
  • this is an attributive adjective
• porselenet er gammelt = the porcelain is old
  
  • here the adjective comes after the verb
  • this is a predicative adjective

A very important point:

• attributive definite adjectives usually take the -e form: det gamle porselenet
• predicative adjectives do not take that definite -e form: porselenet er gammelt

So gammelt is correct here.

Because Norwegian often expresses the by adding an ending to the noun.

So instead of saying a separate word like in English:

• English: the porcelain
• Norwegian: porselenet

This is one of the most common differences between English and Norwegian.

It can be understood in more than one way depending on context.

Porselen can refer to:

• porcelain as a material
• china / porcelain items such as plates, cups, etc.

So Porselenet er gammelt could mean:

• the porcelain material/object is old
• the china set is old

Context tells you which meaning is intended.

Because this is a normal Norwegian main clause with standard word order:

• Porselenet = subject
• er = verb
• gammelt = adjective/complement

So the structure is basically:

Subject + verb + complement

This is very similar to English:

• The porcelain is old.

No, that would sound wrong in standard Norwegian.

Because porselenet is neuter, the adjective must be gammelt, not gammel.

So:

• gammel = common gender singular
• gammelt = neuter singular
• gamle = plural or many definite/adjective-before-noun situations

Correct sentence:

• Porselenet er gammelt.

Usually it means old in age, but the exact nuance depends on context.

With objects, gammelt can mean:

• old in age
• antique
• not new
• old-fashioned, in some contexts

So Porselenet er gammelt could simply mean the porcelain is not new, or it could imply it is quite old or antique.

A rough pronunciation guide is:

• Porselenet ≈ por-se-ლეh-net
• er ≈ ær or a very light e/r sound, depending on dialect
• gammelt ≈ GAM-melt

A few helpful notes:

• the r in Norwegian is usually clearly pronounced, though it varies by dialect
• the -et ending in porselenet is pronounced, but often lightly
• the t in gammelt is usually heard

Exact pronunciation varies a lot across Norway, but this gives you a reasonable starting point.

Yes, this is normal Bokmål and it sounds natural.

It is a simple and correct sentence for saying that the porcelain/china is old.

A Norwegian speaker would understand it easily.