Þetta er algengt meðal barna.

A word-by-word breakdown:

• Þetta = this. Here it is the subject of the sentence.
• er = is, the present tense of vera (to be).
• algengt = common. This is an adjective describing þetta.
• meðal = among.
• barna = children, in the genitive plural form.

So the structure is:

Þetta + er + algengt + meðal barna
This + is + common + among children

Þetta is one form of the demonstrative þessi (this).

Icelandic demonstratives change for gender, number, and case. Here, the word stands on its own, without a noun after it, and it is in the neuter singular form:

• masculine: þessi
• feminine: þessi
• neuter: þetta

So þetta is the correct form when you mean this in a general or abstract sense, like this thing / this situation / this behavior.

Because Icelandic adjectives agree with the word they describe.

Here, algengt is a predicate adjective after er, and it agrees with þetta, which is neuter singular.

The adjective algengur (common) has different forms:

• masculine singular: algengur
• feminine singular: algeng
• neuter singular: algengt

Since þetta is neuter singular, the adjective must also be neuter singular: algengt.

Because the preposition meðal takes the genitive case.

The noun barn (child) changes like this in the plural:

• nominative plural: börn
• genitive plural: barna

So:

• börn = children as a subject or basic dictionary-style plural
• barna = of children / used after certain words and prepositions

Since meðal requires the genitive, you get meðal barna.

Not really in the normal English possessive sense.

Even though barna is a genitive form, Icelandic uses the genitive for more than possession. After meðal, it simply means among children.

So in this sentence, barna does not mean something like the children's in a possessive way. It is just the form required by the preposition.

Meðal takes the genitive.

That is why you say:

• meðal barna = among children
• meðal nemenda = among students
• meðal vina = among friends

This is something you mostly just have to learn with the preposition: meðal + genitive.

Because barna here means children in general, not the children.

So:

• meðal barna = among children, among children in general
• meðal barnanna = among the children

The ending -anna adds the definite article in the genitive plural.

So if you mean children as a group in general, barna is exactly right.

Yes, very often.

In Icelandic, þetta can point to:

• a thing
• an event
• a situation
• a habit
• a behavior
• something just mentioned in conversation

So Þetta er algengt meðal barna can mean:

• This is common among children
• This kind of thing is common among children
• This behavior is common among children

It does not have to refer to a concrete object.

The hardest part for many English speakers is þ.

• þ is like the th in thing
• e is like e in bet
• the double tt is not just a simple English t; in Icelandic it is often pronounced with a little breath before it, so it can sound a bit like thEH-hta or THET-ta to English ears

A rough learner-friendly pronunciation is:

THET-ta

That is only approximate, but it is a useful starting point.

The sentence as given is the most neutral order:

Þetta er algengt meðal barna.

But Icelandic can move parts of the sentence for emphasis. For example:

Meðal barna er þetta algengt.

That puts more focus on among children.

However, Icelandic still follows its usual verb-second pattern, so if you move meðal barna to the front, the verb er still comes next.

Yes. Meðal is a standard way to say among.

In many contexts it is a very natural choice, especially when talking about a group in a general sense, like:

• meðal barna = among children
• meðal fullorðinna = among adults

You may also see á meðal, which can also mean among. But in a sentence like this, meðal barna is completely normal and correct.