Rofinn virkar stundum ekki, svo rafvirkinn þarf líklega að skipta honum út.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word like English the.

• rofi = a switch / breaker

• rofinn = the switch / breaker

• rafvirki = an electrician
• rafvirkinn = the electrician

In this sentence, both are singular, masculine, nominative, because they are the subjects of their clauses.

Here it is a noun: rofi + the definite ending, so rofinn = the switch / the breaker.

That is a good question, because rofinn can also look like a past participle/adjective in other contexts. But in this sentence, the meaning and structure show that it is the noun.

The verb virka means to work, function, operate.

It is the normal verb for devices, systems, plans, medicine, and so on:

• Síminn virkar ekki. = The phone doesn’t work.
• Þetta virkar. = This works.

So here Rofinn virkar stundum ekki means the breaker/switch sometimes doesn’t function.

This is normal Icelandic word order.

• Rofinn = subject
• virkar = finite verb
• stundum = sometimes
• ekki = not

So the clause is literally:

• The breaker works sometimes not

but naturally: The breaker sometimes doesn’t work.

Also, Icelandic is a verb-second language. If you moved stundum to the front, you would get:

• Stundum virkar rofinn ekki.

Not:

• Stundum rofinn virkar ekki.

So the finite verb still has to come early in the clause.

Here svo means so, therefore, or as a result.

It connects the two clauses:

• the breaker sometimes doesn’t work,
• so the electrician probably needs to replace it.

It is a very common everyday connector.

Because Icelandic uses þurfa að + infinitive for need to / have to.

So:

• þarf að skipta = needs to replace / needs to swap out

In the sentence:

• rafvirkinn þarf líklega að skipta honum út
• the electrician probably needs to replace it

Líklega means probably.

It is an adverb modifying the whole idea of þarf að skipta honum út. In other words, the speaker is not completely certain, but thinks replacement is likely.

Its position here is very natural:

• rafvirkinn þarf líklega að skipta honum út

That is roughly:

• the electrician probably needs to replace it

The expression skipta ... út means replace, swap out, or change out.

Literally, it is close to:

• to switch/change it out

So:

• að skipta honum út = to replace it

This is a very common Icelandic verb-particle expression.

Because skipta einhverju út takes the dative case.

The noun being referred to is rofi, which is masculine, so the matching singular dative pronoun is honum.

Very roughly:

• nominative: hann
• accusative: hann
• dative: honum

So Icelandic is not just choosing a word for it; it is also showing the case required by the verb expression.

That is simply how the expression works in Icelandic: skipta einhverju út = replace something, with the thing replaced in the dative.

So if you used the full noun instead of the pronoun, you would get:

• að skipta rofanum út = to replace the switch/breaker

Here rofanum is the dative form of rofinn / rofi.

This is one of those patterns that learners usually just need to memorize together with the verb.

Because in this construction, the object normally comes between the verb and the particle:

• skipta honum út
• skipta rofanum út

So the particle út stays at the end of the phrase.

This is very similar to English swap it out, where it also comes before out.