Í þessu orði eru tvö sérhljóð og þrjú samhljóð.

Because í can take two different cases in Icelandic:

• dative for location: in
• accusative for motion into something: into

Here the sentence is describing what is in the word, not movement into the word, so Icelandic uses the dative:

• í þessu orði = in this word

That is why both words change form:

• þetta → þessu
• orð → orði

Þessu is a form of þessi meaning this. It has to agree with the noun it goes with in:

• gender
• number
• case

The noun orð is:

• neuter
• singular
• dative here (because of í)

So the correct form is þessu.

A useful mini-pattern is:

• þetta orð = this word
• í þessu orði = in this word

The basic dictionary form is orð. But after í in a location meaning, the noun goes into the dative singular, and the dative singular form is orði.

So:

• orð = word
• orði = to/in a word, depending on context

In this sentence, it means in this word.

This is normal Icelandic word order. Icelandic is often described as a verb-second language. That means that when something other than the subject comes first, the finite verb usually comes next.

Here the sentence begins with the prepositional phrase:

• Í þessu orði = In this word

So the verb comes next:

• Í þessu orði eru ...

After that comes the subject:

• tvö sérhljóð og þrjú samhljóð

In more English-like order, the structure is roughly:

• In this word are two vowels and three consonants

That sounds formal or poetic in English, but it is normal in Icelandic.

Because the thing that exists in the word is plural:

• tvö sérhljóð og þrjú samhljóð = two vowels and three consonants

That whole noun phrase is plural, so the verb is plural too:

• eru = are

If there were only one thing, you would use er = is.

In Icelandic, the numbers two, three, and four change form depending on gender and sometimes case.

Both sérhljóð and samhljóð are neuter nouns, so the neuter forms are used:

• tvö = two (neuter)
• þrjú = three (neuter)

Compare:

• masculine: tveir, þrír
• feminine: tvær, þrjár
• neuter: tvö, þrjú

So here:

• tvö sérhljóð
• þrjú samhljóð

Because many Icelandic neuter nouns have the same form in the nominative/accusative singular and plural.

So:

• sérhljóð can mean a vowel or vowels
• samhljóð can mean a consonant or consonants

The number word tells you which meaning is intended:

• eitt sérhljóð = one vowel

• tvö sérhljóð = two vowels

• eitt samhljóð = one consonant
• þrjú samhljóð = three consonants

Yes. Icelandic often expresses there is / there are without a separate dummy word like English there.

So instead of something literally matching English there are, Icelandic simply says:

• Í þessu orði eru tvö sérhljóð og þrjú samhljóð

Literally, that is closer to:

• In this word are two vowels and three consonants

But the natural English meaning is:

• There are two vowels and three consonants in this word

A few key ones:

• þ is like th in think
• ð is usually like th in this, though in real speech it can be softer or weaker
• í is a long ee sound
• ö is a rounded vowel with no exact English equivalent
• hl at the start of a word is a special Icelandic cluster; for a beginner, saying a clear l is an acceptable first step

A rough beginner-friendly pronunciation guide could be:

• Í þessu orði eru tvö sérhljóð og þrjú samhljóð

Approximate:

• ee THESS-u OR-thi EH-ru TVOE SEHR-hlyoth og THRYU SAM-hlyoth

That is only approximate, but it helps you get started.

They are compound words:

• hljóð = sound
• sér- = separate, special, individual
• sam- = together, with

So:

• sérhljóð is the Icelandic word for vowel
• samhljóð is the Icelandic word for consonant

This matches the traditional idea that:

• a vowel can sound on its own
• a consonant sounds together with a vowel

Yes. This is a very useful pattern for describing what something contains.

Pattern:

• Í + dative noun phrase + er/eru + number + noun

Examples:

• Í húsinu eru þrír gluggar.
  There are three windows in the house.

• Í setningunni eru fimm orð.
  There are five words in the sentence.

• Í nafninu eru tvö sérhljóð.
  There are two vowels in the name.

The main things to watch are:

• use dative after í for location
• choose er or eru
• make sure the number agrees correctly where needed