Bordet er solid.

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

• bord = table
• bordet = the table

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

In a simple noun phrase, Norwegian often uses a suffix instead of a separate word.

So:

• bordet = the table

You do sometimes see a separate definite word too, but usually when there is an adjective before the noun:

• det solide bordet = the solid table

That pattern is often called double definiteness.

You usually learn a Norwegian noun together with its article:

• et bord = a table

The article et tells you it is neuter. That helps you form other versions:

• et bord = indefinite singular
• bordet = definite singular

So it is a good habit to memorize nouns as et bord, not just bord.

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

It works like English is / am / are, but Norwegian uses the same form er for all persons:

• jeg er
• du er
• han er
• bordet er

So in this sentence, er simply links the subject bordet with the description solid.

Because here solid is a predicate adjective. It comes after er and describes the subject.

• Bordet er solid. = the adjective comes after the verb
• et solid bord = the adjective comes before the noun

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

In Norwegian, adjectives often agree with the noun, including after er. However, solid is one of the adjectives that normally keeps the same form in singular common gender and singular neuter.

So you get:

• en solid stol
• et solid bord
• Bordet er solid

In other forms, you will often see:

• solide bord

So yes, solid is the correct form here.

No, this is the normal basic order for a statement:

• subject + verb + complement

So:

• Bordet = subject
• er = verb
• solid = complement/adjective

If you turn it into a yes/no question, Norwegian usually puts the verb first:

• Er bordet solid?

No, not in a normal full sentence. Norwegian usually needs a finite verb, just like English does.

So:

• Bordet er solid. = correct
• Bordet solid. = not a normal complete sentence

The verb er is required.

You would say:

• et solid bord

That is different from:

• Bordet er solid.

The first is a noun phrase: a solid table.
The second is a full sentence: The table is solid.

This is a very useful contrast to learn:

• et solid bord
• bordet er solid

Yes, the structure is very common:

• Stolen er solid. = The chair is solid.
• Huset er solid. = The house is solid.

You just need to make sure the noun form is correct, especially its gender and definite ending. The pattern itself is simple and very common:

• [definite noun] + er + [adjective]