Jeg lægger noten i mappen, så jeg ikke mister den.

Because at lægge means to put / to lay something somewhere, while at ligge means to lie / to be located somewhere.

In this sentence, the speaker is actively placing the note into the folder, so Danish uses lægger:

Jeg lægger noten i mappen = I put the note in the folder

Compare:

Noten ligger i mappen = The note is lying / is in the folder

So a useful shortcut is:

• lægge = putting something somewhere
• ligge = being somewhere

That -en is the definite article attached to the noun. In Danish, instead of putting a separate word like the before the noun, you often add the article to the end.

So:

• en note = a note

• noten = the note

• en mappe = a folder
• mappen = the folder

Here, the speaker means a specific note and a specific folder, so the definite forms are used.

Here så means so or so that. It introduces the reason or purpose for the action in the first clause.

So:

Jeg lægger noten i mappen, så jeg ikke mister den.

means:

I put the note in the folder so that I don't lose it.

In everyday Danish, this is a natural way to express purpose.

Because after så, you have a subordinate clause, and Danish word order changes in subordinate clauses.

In a subordinate clause, words like ikke usually come before the finite verb:

• så jeg ikke mister den

This is a very important Danish pattern.

Compare:

Main clause: Jeg mister den ikke. = I’m not losing it / I don’t lose it

Subordinate clause: ... så jeg ikke mister den.

So the position of ikke helps show that this is a subordinate clause.

Because note is a common-gender noun: en note, not et note.

In Danish, pronouns usually match the gender of the noun:

• en-note words → den
• et-word words → det

So:

noten → den

That is why the sentence says:

... så jeg ikke mister den.

If the noun were neuter, you would use det instead.

Yes. The infinitive is at miste = to lose, and mister is the present tense:

• at miste = to lose
• jeg mister = I lose / I am losing

Danish verbs do not change for person the way English verbs sometimes do. So you get:

• jeg mister
• du mister
• han/hun mister
• vi mister

The form stays the same for all persons.

Yes. A very common alternative is:

Jeg lægger noten i mappen for ikke at miste den.

That means essentially the same thing: I put the note in the folder so I don't lose it.

The difference is mainly structure:

• så jeg ikke mister den = so that I don't lose it
• for ikke at miste den = in order not to lose it

Both are natural, but the version with for ikke at is especially common when the subject is the same in both parts.

The comma separates the first clause from the clause that explains the purpose/result:

Jeg lægger noten i mappen, så jeg ikke mister den.

So the comma helps show the structure:

• first clause: Jeg lægger noten i mappen
• second clause: så jeg ikke mister den

In Danish, commas are often used to mark clause boundaries like this.

Mappe usually means something like folder, file, or sometimes binder, depending on context.

So i mappen means the note is being put into some kind of folder or file where it can be kept safely.

The exact English word can vary a little depending on the situation, but folder is usually the best basic translation here.