Það er mikil gleði í brúðkaupinu.

In this sentence, það er works like English there is.

So Það er mikil gleði... is literally something like There is great joy...

Icelandic often uses það er to introduce the existence or presence of something, especially when the real subject comes later in the sentence.

• það = it/that
• er = is

But here you should understand the whole expression það er as an existential pattern, not as a literal it is.

Because mikil has to agree with gleði.

The noun gleði is:

• feminine
• singular
• nominative here

So the adjective mikill must also appear in the feminine singular nominative form:

• masculine: mikill
• feminine: mikil
• neuter: mikið

That is why the sentence has mikil gleði.

Gleði is in the nominative case.

That is because it is the main noun being introduced by the existential structure það er. In a sentence like this, the thing that exists / is present is usually in the nominative.

So:

• Það er mikil gleði = There is great joy

Here, gleði is the thing being said to exist.

Because í can take either the dative or the accusative, depending on meaning.

• í + dative = in/at a location, no movement
• í + accusative = into something, movement toward it

Here the sentence describes where the joy is happening, not movement into the wedding. So Icelandic uses the dative:

• í brúðkaupinu = at/in the wedding

If there were movement, you might expect the accusative instead.

The ending -inu includes the definite article, so brúðkaupinu means the wedding.

The base noun is:

• brúðkaup = wedding

In this sentence it is:

• singular
• dative
• definite

Icelandic usually adds the as an ending on the noun instead of using a separate word.

So:

• brúðkaup = wedding
• brúðkaupinu = the wedding (in the dative form)

Because Icelandic usually puts the definite article onto the noun itself as a suffix.

English says:

• the wedding

Icelandic usually says:

• brúðkaupið or brúðkaupinu, depending on case

So instead of a separate article before the noun, Icelandic often attaches it to the end.

That is why you see brúðkaupinu, not a separate word meaning the before it.

Because gleði here is being used as a general or uncountable idea: joy.

English also often does this:

• There is joy in the house
• There is great happiness at the party

You do not have to say the joy unless you mean a specific, already identified joy.

So mikil gleði means great joy / a lot of joy, not the joy.

It can suggest either, depending on context.

mikill / mikil / mikið often means:

• great
• much
• a lot of

So mikil gleði can be understood as:

• great joy
• much joy
• a lot of joy

In natural English, There is great joy at the wedding is often the smoothest translation, but the Icelandic phrase also carries the sense of a lot of joy being present.

Because the noun being talked about, gleði, is singular.

Even though joy is an abstract idea, in grammar it is treated as a singular noun here.

So Icelandic uses:

• er = is

If the noun were plural, you would use the plural verb form instead.

Yes, Icelandic word order is more flexible than English, though not completely free.

The basic sentence is:

• Það er mikil gleði í brúðkaupinu.

But Icelandic can move parts around for emphasis. For example, the location might come earlier in some contexts.

Still, the version you have is a very natural neutral sentence for a learner to study.

A useful point is that Icelandic often keeps það er at the start in this kind of existential sentence.

In natural English, at the wedding is usually better.

Even though í often literally means in, Icelandic and English do not always match word-for-word in prepositions.

So:

• literal-ish: in the wedding
• natural English: at the wedding

A learner should remember the Icelandic form í brúðkaupinu, but translate it idiomatically in English when appropriate.

Brúðkaup is a neuter noun, and yes, that matters because it affects its forms.

In this sentence, after í with a location meaning, the noun must be in the dative, and the definite article must match that form too. That is part of why you get brúðkaupinu.

So gender matters because Icelandic noun endings and article endings depend on:

• gender
• number
• case

A good practical rule is:

• location/state → usually dative
• motion into something → usually accusative

Here the sentence describes a situation already happening at the wedding. Nothing is moving into anything. So the dative is used:

• í brúðkaupinu

This dative/accusative contrast is very common in Icelandic with prepositions such as í and á, so it is worth learning early.