Moltan er góð.

In Icelandic, the definite article is very often attached to the end of the noun instead of written as a separate word.

So here:

molta + definite ending -n → moltan

That is why Moltan means the molta. This is very common in Icelandic. For example:

kona → konan
saga → sagan

The dictionary form is molta.

Icelandic dictionaries usually list nouns in the indefinite nominative singular. So if you look the word up, you would normally search for molta, not moltan.

er means is.

It is the 3rd person singular present tense form of the verb vera, which means to be.

So:

• vera = to be
• er = is

Because the adjective has to agree with the noun.

Here, moltan is:

• feminine
• singular
• nominative (because it is the subject)

So the adjective must also be feminine singular nominative.

The basic forms are:

• góður = masculine
• góð = feminine
• gott = neuter

So with a feminine singular subject, góð is the correct form.

Because góð is the form normally used for a predicate adjective after er.

In this sentence, góð is not directly in front of the noun; it comes after er and describes the subject:

Moltan er góð. = The molta is good.

That is different from an adjective placed directly before a definite noun, where you often get a weak form such as góða. So compare:

• Moltan er góð. = The molta is good.
• góða moltan = the good molta

It is in the nominative case.

The nominative is used for the subject of the sentence, and here Moltan is the subject of er góð.

Because the subject is nominative, the predicate adjective góð also appears in the matching nominative form.

Yes. This sentence uses a very straightforward word order:

Moltan — subject
er — verb
góð — predicate adjective

So it works much like English The molta is good.

Icelandic word order can be more flexible than English in some contexts, but this sentence is a very normal, basic pattern.