Hvis knappen går af min skjorte, lægger jeg den i min lomme, så jeg ikke mister den.

Because the sentence starts with a subordinate clause: Hvis knappen går af min skjorte.

In Danish main clauses, the finite verb normally comes second. So when the if-clause comes first, it takes the first position, and the verb of the main clause moves in front of the subject:

• Hvis knappen går af min skjorte, lægger jeg den i min lomme.

If you started directly with the main clause, you would say:

• Jeg lægger den i min lomme, hvis knappen går af min skjorte.

So this is a standard example of Danish V2 word order.

Here går af means comes off or comes loose.

So:

• knappen går af min skjorte = the button comes off my shirt

This is a very natural Danish expression for something attached becoming detached.

A related verb is falder af, which means falls off. In many situations both are possible, but går af is especially common for things like buttons coming off clothing.

Because Danish uses different ways to mark definiteness.

• knappen = the button
• skjorte = shirt
• min skjorte = my shirt

With a possessive like min, Danish normally does not add the definite ending as well. So:

• min skjorte = correct
• min skjorten = not correct

Meanwhile, knappen is definite because it refers to a specific button in the situation.

Because knap is a common gender noun in Danish, and the common-gender pronoun is den.

So:

• en knap → den
• et hus → det

In this sentence, both instances of den refer back to knappen:

• lægger jeg den i min lomme = I put it in my pocket
• så jeg ikke mister den = so I don’t lose it

Because så jeg ikke mister den is a subordinate clause.

In Danish subordinate clauses, words like ikke usually come before the finite verb:

• så jeg ikke mister den

Compare that with a main clause, where ikke usually comes after the finite verb:

• Jeg mister den ikke.

So this is a very important word-order contrast:

• Main clause: verb + ikke
• Subordinate clause: ikke
  • verb

Here så means so that.

So:

• så jeg ikke mister den = so that I don’t lose it

It introduces a clause explaining the purpose or result.

A close alternative would be:

• for ikke at miste den = in order not to lose it

So these are both natural:

• ... så jeg ikke mister den
• ... for ikke at miste den

The version with så uses a full clause with a subject and finite verb. The version with for at uses an infinitive structure.

Because Danish often uses the present tense for:

• general truths
• habits
• typical situations
• future meaning when the context is clear

This sentence describes a typical future situation:

• Hvis knappen går af min skjorte, lægger jeg den i min lomme...

English often does something similar:

• If the button comes off my shirt, I put it in my pocket...
• or more naturally in meaning, I’ll put it in my pocket

Danish does not need a special future form here.

Because i means in, and a pocket is treated as something you put an object inside.

So:

• i min lomme = in my pocket

Also, just like with min skjorte, the possessive min replaces the article:

• min lomme = my pocket
• not min lommen

Yes, in many contexts you could say falder af, and it would mean falls off.

But there is a nuance:

• går af often suggests comes off / becomes detached
• falder af emphasizes the actual falling off

With a button, går af is very idiomatic and common. It focuses on the fact that the button has come loose from the shirt, not just the physical motion.

Because Danish usually puts the definite article at the end of the noun instead of using a separate word.

So:

• en knap = a button
• knappen = the button

This is one of the biggest differences from English. Danish often uses an attached ending for the:

• skjorte → skjorten
• lomme → lommen
• knap → knappen

But after a possessive like min, you do not use that ending:

• min skjorte
• min lomme

It is a natural Danish sentence.

Everything in it is normal and idiomatic:

• Hvis for if
• går af for a button coming off
• lægger jeg because of Danish main-clause word order
• så jeg ikke mister den for so that I don’t lose it

A Dane would understand it immediately, and it sounds like ordinary everyday Danish.