Rútínan mín breytist oft.

Because -n here is the definite article attached to the noun.

• rútína = routine
• rútínan = the routine

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

So:

• rútína = a routine / routine
• rútínan mín = my routine

Even though English does not say the my routine, Icelandic normally does use the definite form with a following possessive:

• bókin mín = my book
• húsið mitt = my house
• rútínan mín = my routine

Because that is a very common and natural Icelandic word order for possessives.

Icelandic often says:

• rútínan mín = my routine
• vinur minn = my friend
• fjölskyldan mín = my family

So the pattern is often:

noun + definite article + possessive

This is different from English, where my comes before the noun.

A form like mín rútína can exist, but it is more marked or stylistic and is not the neutral everyday phrasing here. In a normal sentence, rútínan mín is what learners should expect.

Mín is the feminine singular nominative form of the possessive pronoun meaning my.

It matches rútínan, which is:

• feminine
• singular
• nominative (because it is the subject of the sentence)

So the agreement is:

• rútínan → feminine singular nominative
• mín → feminine singular nominative

If the noun were a different gender or case, the possessive form would change too.

For example:

• bíllinn minn = my car (masculine)
• húsið mitt = my house (neuter)

Breytist is the 3rd person singular present tense of breytast, which means to change or to undergo change.

Here the subject is rútínan mín = my routine, which is a singular third-person subject, so the verb is in the matching form:

• ég breytist = I change
• þú breytist = you change
• hann/hún/það breytist = he/she/it changes

So:

• Rútínan mín breytist = My routine changes

The -st is a very important Icelandic ending. It often marks a middle voice form.

In many cases, -st verbs are used where English has an intransitive idea such as:

• change
• move
• open
• become visible
• happen by itself / undergo something

Here the difference is roughly:

• breyta = to change something
  
  • Ég breyti rútínunni. = I change the routine.
• breytast = to change / to undergo change
  
  • Rútínan breytist. = The routine changes.

So in your sentence, the routine is not changing something else; it is itself changing. That is why breytast is used.

It is in the nominative case, because it is the subject of the sentence.

The sentence structure is:

• Rútínan mín = subject
• breytist = verb
• oft = adverb

Since the subject is nominative, both the noun phrase and the possessive agree in nominative form:

• rútínan
• mín

This is one of the main things to watch in Icelandic: adjectives, articles, and possessives often change form depending on case, gender, and number.

Because that is a normal place for an adverb like oft (often) in a simple declarative sentence.

So:

• Rútínan mín breytist oft. = My routine often changes.

In Icelandic, adverbs of frequency often come after the finite verb, especially in straightforward main clauses.

That said, word order can sometimes change for emphasis or style. But this sentence has a very natural, neutral order.

Not as a normal neutral statement.

In ordinary Icelandic main-clause word order, the finite verb usually comes early, and oft would normally follow it here:

• Rútínan mín breytist oft.

Putting oft before breytist in this sentence would sound unusual or marked. A learner should stick with breytist oft in this kind of sentence.

A rough learner-friendly pronunciation is:

ROO-tee-nan meen BRAY-tist oft

A few helpful points:

• ú sounds roughly like oo
• í sounds roughly like ee
• ey in breytist sounds like ay
• st at the end of a word is pronounced clearly

So:

• Rútínan ≈ ROO-tee-nan
• mín ≈ meen
• breytist ≈ BRAY-tist
• oft ≈ oft

As always, the exact Icelandic sounds are a bit different from English, but this gets you close.

Yes. It is a natural and correct sentence.

It sounds like everyday Icelandic and follows common patterns:

• definite noun + possessive: rútínan mín
• middle/intransitive verb: breytist
• adverb of frequency after the verb: oft

So it is a very good model sentence for learners.