Butikken hadde ikke min størrelse, så jeg måtte prøve en annen jakke i prøverommet.

Both are past tense forms.

• ha → hadde
• måtte is the past tense of må

So the sentence is describing something that happened in the past:

• the shop did not have the right size
• as a result, the speaker had to try another jacket

In English, måtte usually corresponds to had to, not must, when talking about the past.

In a normal main clause, Norwegian usually places ikke after the finite verb.

So:

• Butikken hadde ikke min størrelse

follows the common pattern:

• subject + finite verb + ikke + rest

You can compare:

• Jeg forstår ikke.
• Han kommer ikke i dag.
• Vi hadde ikke tid.

This is one of the most important word-order patterns in Norwegian.

Both are possible, but they are structured differently.

• min størrelse = my size with the possessive before the noun
• størrelsen min = my size with the possessive after the noun

In Norwegian:

• If the possessive comes before the noun, the noun is usually indefinite: min størrelse
• If the possessive comes after the noun, the noun is usually definite: størrelsen min

In everyday Norwegian, størrelsen min is often more common and natural in speech.
Min størrelse can sound a bit more marked, formal, or emphatic, but it is still correct.

Because Norwegian normally does not use an article when the possessive comes before the noun.

So you say:

• min størrelse
• mitt hus
• mine sko

not:

• en min størrelse
• et mitt hus
• de mine sko

The possessive itself already does the job that the article would otherwise help with.

Here, så means so, and it is acting as a coordinating conjunction joining two main clauses.

That means the second clause keeps normal main-clause word order:

• så jeg måtte prøve ...

Inside that clause, jeg is the subject and måtte is the finite verb.

If you wrote så måtte jeg ..., that could sound like then I had to ..., where så is being understood more as an adverb meaning then rather than as the conjunction so.

So in this sentence, så jeg måtte is the natural choice.

Because måtte is a modal verb, and modal verbs in Norwegian are followed by the bare infinitive, without å.

So you get:

• jeg måtte prøve
• jeg kan komme
• jeg vil spise
• du skal gå

not:

• jeg måtte å prøve

This is very similar to English, where we say I had to try, not I had to to try.

Here it means try on.

In clothing contexts, prøve often works the way English try on does:

• prøve en jakke = try on a jacket

The context makes it clear that the speaker is putting on the jacket to see if it fits.

Norwegian often does not need a separate word like English on in this kind of sentence.

Because annen has to agree with the noun.

Here:

• jakke is a singular common-gender noun
• so you use en annen jakke

The forms are:

• annen for singular common gender
• annet for singular neuter
• andre for plural and some definite contexts

Examples:

• en annen jakke
• et annet rom
• andre sko

A useful extra note: jakke is historically feminine, so in Bokmål you may also see feminine-style forms such as ei annen jakke or ei anna jakke, depending on style. But en annen jakke is completely normal.

Prøverommet is a compound noun:

• prøve = try
• rom = room

Together, prøverom means fitting room or changing room.

Norwegian writes compound nouns as one word, much more often than English does. So where English has fitting room, Norwegian has prøverom.

The ending -met makes it definite:

• et prøverom = a fitting room
• prøverommet = the fitting room

Because i is the normal preposition for being in/inside a room.

So:

• i prøverommet = in the fitting room

That is the natural choice when someone is physically inside that space.

The definite form prøverommet is also natural because the sentence refers to the fitting room in the shop—the specific one the speaker used.

Yes. A very natural alternative would be:

• Butikken hadde ikke størrelsen min, så jeg måtte prøve en annen jakke i prøverommet.

This version uses størrelsen min instead of min størrelse, which many learners will hear more often in everyday speech.

Both are correct, but:

• min størrelse can feel a bit more formal or emphatic
• størrelsen min often feels more conversational

So if you're aiming for everyday spoken Norwegian, størrelsen min is a very useful pattern to remember.