Samkvæmt reglunni er þetta bannað.

Samkvæmt means according to or in accordance with.

In this sentence, it is used with the dative case, so the noun after it has to be in dative form:

• regla = rule
• samkvæmt reglunni = according to the rule

So a very useful pattern is:

samkvæmt + dative

Examples:

• samkvæmt lögunum = according to the laws
• samkvæmt samningnum = according to the agreement

Because samkvæmt requires the dative, and the sentence is talking about the rule, not just a rule.

The forms are:

• regla = a rule, nominative
• reglan = the rule, nominative
• reglu = a rule, dative
• reglunni = the rule, dative

So after samkvæmt, the correct form here is reglunni.

In Icelandic, the definite article is usually attached to the end of the noun rather than written as a separate word.

So:

• regla = rule
• reglan = the rule
• reglunni = to/according to the rule, with dative ending and definite article combined

This is very normal in Icelandic. English uses a separate word, but Icelandic often builds the into the noun itself.

Because Icelandic usually follows a verb-second pattern in main clauses.

That means the finite verb tends to come in the second position of the sentence, even if something other than the subject comes first.

Here, the first element is the prepositional phrase:

• Samkvæmt reglunni = according to the rule

So the verb comes next:

• er = is

Then the subject follows:

• þetta = this

Structure:

• Samkvæmt reglunni | er | þetta | bannað

This is a very common Icelandic word-order pattern.

Þetta means this.

Here it is a neuter singular demonstrative pronoun. It refers to some thing, action, situation, or idea already understood from context.

For example, if someone points at an action or proposal, they can say:

• Þetta er bannað = This is forbidden

So þetta is the subject of the sentence.

Bannað is the neuter singular form of bannaður, which means forbidden or prohibited.

It is neuter singular because it agrees with þetta, which is also neuter singular.

So:

• þetta = neuter singular
• bannað = neuter singular

This kind of agreement is very important in Icelandic. Predicate adjectives and participles often match the gender and number of the subject.

It is best understood here as a past participle used adjectivally.

The verb is banna = to forbid / to ban.

From that you get forms like:

• bannaður = forbidden, banned
• bönnuð = forbidden, banned, feminine
• bannað = forbidden, banned, neuter

In er þetta bannað, Icelandic uses vera + participle/adjectival form in a way that corresponds to English is forbidden.

So in practice, you can think of bannað here as meaning forbidden.

Yes. That is also correct.

Both sentences are grammatical:

• Samkvæmt reglunni er þetta bannað.
• Þetta er bannað samkvæmt reglunni.

The difference is mainly one of focus or information structure:

• Samkvæmt reglunni... puts according to the rule first, so it frames the whole statement from the rule’s point of view.
• Þetta er bannað... starts with this, which is a more neutral subject-first order.

So the original sentence is perfectly natural, but not the only possible word order.

You would use the dative plural:

• reglur = rules
• reglunum = the rules, dative plural

So:

• Samkvæmt reglunum er þetta bannað. = According to the rules, this is forbidden.

This is a good example of the same pattern:

samkvæmt + dative

Whether singular or plural, the noun after samkvæmt still has to be in the dative case.