Uniformen er ren.

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

• en uniform = a uniform
• uniformen = the uniform

So -en here is the definite ending.

In Norwegian, the is often built into the noun instead of written as a separate word.

So instead of something like the uniform, Norwegian says uniformen.

This is very common in Scandinavian languages:

• en bil = a car
• bilen = the car

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

So:

• jeg er = I am
• du er = you are
• uniformen er = the uniform is

It works much like English is/am/are, except Norwegian uses er for all persons in the present tense.

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

Here, uniformen is a singular en-word noun, so the adjective takes the basic form:

• ren for singular en-word
• rent for singular neuter (et-word)
• rene for plural

Examples:

• uniformen er ren = singular en-word
• rommet er rent = singular neuter
• klærne er rene = plural

Because this adjective is being used after the verb er, as a predicative adjective. In that position, Norwegian adjectives agree for gender and number, but not for definiteness in the same way they do before a noun.

So:

• uniformen er ren = correct
• not uniformen er rene

Compare:

• den rene uniformen = the clean uniform
  Here the adjective comes before the noun, so it takes the definite/plural-style form rene.
• uniformen er ren = the uniform is clean
  Here the adjective comes after er, so it stays ren.

Yes, in this sentence it is treated as an en-word.

That gives:

• en uniform = a uniform
• uniformen = the uniform

This matters because the adjective form matches that noun type:

• en uniform er ren

No, this is very straightforward Norwegian word order:

• Uniformen = subject
• er = verb
• ren = adjective/complement

So it follows the same basic pattern as English:

The uniform + is + clean

Ren is pronounced roughly like rain in many English accents, but not exactly.

A few tips:

• the r is Norwegian, not English
• the vowel is long
• the final n is clearly pronounced

A rough approximation for English speakers is rane without fully saying a separate second syllable.

Yes. Ren can also mean pure in some contexts.

For example:

• rent vann = pure/clean water
• ren energi = clean energy

In Uniformen er ren, the natural meaning is clean.

Yes, but the meaning changes slightly.

• Uniformen er ren = the uniform is clean
  You are talking about a specific uniform.
• En uniform er ren = a uniform is clean
  This sounds more general or less specific.

So if you mean one particular uniform, uniformen is the right choice.