Min paraply er ødelagt, så jeg må købe en ny.

Because paraply is a common-gender noun, also called an en-word: en paraply.

In Danish possessives match the gender/number of the noun:

• min for singular en-words
• mit for singular et-words
• mine for plurals

So:

• min paraply
• mit hus
• mine sko

Because the possessive min already fills that slot.

Just like English says my umbrella, not my an umbrella, Danish says min paraply, not en min paraply.

So min works like my, and no extra article is needed.

Ødelagt is the past participle of the verb ødelægge (to damage / to break), but in this sentence it is being used like an adjective after er.

So er ødelagt means is broken / is damaged.

This is very common in Danish:

• Døren er lukket = The door is closed
• Bilen er stjålet = The car is stolen
• Min paraply er ødelagt = My umbrella is broken

Because this adjective/participle has the form ødelagt in the singular.

With some Danish adjectives, the forms change more obviously, like:

• ny
• nyt
• nye

But participles like ødelagt often already have the -t form in singular use:

• en ødelagt paraply
• et ødelagt ur
• ødelagte ting (plural)

So here ødelagt is just the correct singular form.

Because the sentence contains two clauses:

• Min paraply er ødelagt
• så jeg må købe en ny

The comma marks the boundary between them. In Danish, this is very normal when one full clause is followed by another.

So the comma helps show: My umbrella is broken, so I have to buy a new one.

Here så means so.

It links a cause and a result:

• My umbrella is broken
• so I have to buy a new one

Be aware that så can mean other things in other contexts, such as then or such/so, but in this sentence it clearly means so.

Because in this sentence så is being used like a conjunction meaning so, linking two clauses. After that kind of så, normal clause order is used:

• så jeg må købe en ny

If you used an adverb like derfor (therefore), Danish would usually invert the order:

• Derfor må jeg købe en ny

And så må jeg købe en ny is also possible, but that sounds more like Then I’ll have to buy a new one or gives a slightly different emphasis.

Må is a modal verb. In this sentence it expresses necessity, similar to must or have to.

So jeg må købe en ny means something like:

• I must buy a new one
• I have to buy a new one

Here the idea is not really permission. It is necessity caused by the situation: the umbrella is broken.

A learner often compares:

• må = must / have to
• kan = can / be able to
• skal = often shall / have to / am supposed to, but with a slightly different feel

Because må is the finite verb in the sentence, and the verb after a modal stays in the infinitive.

So:

• jeg må købe not
• jeg må køber

This is just like English:

• I must buy not
• I must buys

Because after Danish modal verbs, you use the infinitive without at.

So you say:

• jeg må købe
• jeg kan købe
• jeg vil købe
• jeg skal købe

not:

• jeg må at købe

This is a very important pattern in Danish.

Because the noun is understood from the context.

En ny means a new one, with paraply left out because it is already obvious.

So these both work:

• jeg må købe en ny
• jeg må købe en ny paraply

The first one is more natural when the noun has just been mentioned.

Because the missing noun is paraply, and paraply is an en-word in singular.

Adjectives agree with the noun:

• en ny paraply
• et nyt hus
• nye paraplyer

Since the implied noun is singular common gender, ny is correct:

• en ny

These two vowels are difficult for English speakers because English does not have exact matches.

• ø in købe: say something like eh or uh, but with rounded lips
• y in ny: say ee, but with rounded lips

A useful trick:

• for y, try saying English ee while keeping your lips rounded
• for ø, try a more central vowel, also with rounded lips

So:

• købe has a rounded front vowel
• ny has another rounded front vowel, but closer and tenser

You will usually need to hear and imitate native speech here, because spelling alone is not enough.