Jeg var lige ved at glemme kompasset, men min søster lagde det i lommen på min jakke.

Lige ved at + infinitive means to be just about to or to almost do something.

So:

• Jeg var lige ved at glemme kompasset = I was just about to forget the compass / I almost forgot the compass

A few notes:

• lige often adds the idea of just or very nearly
• ved at is part of this fixed expression
• You normally follow it with an infinitive: at glemme, at falde, at grine, etc.

Examples:

• Jeg var lige ved at falde. = I almost fell.
• Hun var lige ved at græde. = She was just about to cry / She almost cried.

Var is the past tense of være (to be).

• jeg er = I am
• jeg var = I was

Since the sentence describes something that happened in the past, Danish uses var:

• Jeg var lige ved at glemme kompasset = I was about to forget the compass

If you said Jeg er lige ved at glemme kompasset, it would mean:

• I am just about to forget the compass
• or more naturally, I’m about to forget the compass right now

In this expression, at introduces the infinitive verb.

• glemme = forget
• at glemme = to forget

The pattern is:

• være lige ved at + infinitive

So:

• Jeg var lige ved at glemme ...
• Han er lige ved at løbe ind i døren.
• Vi var lige ved at tage fejl.

You can think of at here as the Danish equivalent of to in to forget.

Because kompasset is the definite form: the compass.

The noun is:

• et kompas = a compass

Since kompas is a neuter noun (et noun), the definite singular ending is typically -et:

• et kompas = a compass
• kompasset = the compass

So in the sentence:

• glemme kompasset = forget the compass

This is very common in Danish, where the definite article is often attached to the end of the noun rather than written as a separate word.

The basic noun is kompas, but when you add the definite ending -et, Danish spelling gives you:

• kompas
  • -et → kompasset

That double s helps reflect the pronunciation and is simply the correct spelling of the definite form.

So:

• et kompas
• kompasset

This is something you usually just learn as part of the noun’s forms.

Det means it, and it refers back to kompasset.

So:

• kompasset = the compass
• det = it

Why det?

Because kompas is a neuter noun:

• et kompas

Neuter nouns are typically referred to with det in the singular.

Compare:

• en bog → den
• et kompas → det

So:

• Min søster lagde det i lommen ... = My sister put it in the pocket ...

Lagde is the past tense of lægge, which usually means to lay, to put, or to place.

• lægge = to put / lay
• lagde = put / laid

So:

• min søster lagde det i lommen = my sister put it in the pocket

This is a very common Danish verb when something is placed somewhere.

Examples:

• Jeg lagde nøglerne på bordet. = I put the keys on the table.
• Hun lagde bogen i tasken. = She put the book in the bag.

English often uses put in many situations where Danish uses lægge.

This is a very common learner question.

• lægge = to lay / put something somewhere
  
  • usually involves moving something
• ligge = to lie / be lying
  
  • describes where something is

Compare:

• Jeg lagde bogen på bordet. = I put the book on the table.
• Bogen ligger på bordet. = The book is lying on the table.

In your sentence:

• min søster lagde det i lommen = she moved the compass and placed it there

So lagde is correct because there is an action of putting something somewhere.

I means in, and lommen means the pocket.

So:

• i lommen = in the pocket

The noun is:

• en lomme = a pocket
• lommen = the pocket

Danish uses i here because the compass ends up inside the pocket.

Examples:

• i tasken = in the bag
• i skuffen = in the drawer
• i lommen = in the pocket

Here på min jakke means on my jacket or, more naturally in English, of my jacket.

So:

• lommen på min jakke = the pocket on my jacket / the pocket of my jacket

Danish often uses på when something is physically on or attached to something else:

• knappen på skjorten = the button on the shirt
• lommen på jakken = the pocket on the jacket

If you said i min jakke, that would usually mean inside my jacket, which is a different idea.

So:

• i lommen på min jakke = in the pocket on my jacket

Because in Danish, when you use a possessive like min, din, hans, hendes, etc., the noun normally stays in its indefinite form.

So:

• min jakke = my jacket
• not min jakken

This is different from English in form, but it is a basic Danish rule.

Compare:

• jakken = the jacket
• min jakke = my jacket

More examples:

• bogen = the book

• min bog = my book

• huset = the house
• vores hus = our house

So in your sentence:

• på min jakke = on my jacket

Both types of structure can exist in Danish, but lommen på min jakke is a very natural and clear way to say the pocket of/on my jacket.

Danish often expresses possession or part-whole relationships with:

• [definite noun] + på + [owner/object]

So:

• døren til huset = the door of the house
• taget på bilen = the roof of the car
• lommen på min jakke = the pocket on my jacket

A compound noun such as jakkelomme can also exist in Danish and would mean jacket pocket, but the sentence as given is completely normal and perhaps a bit more explicit.

Because men is a coordinating conjunction, like but in English.

After men, Danish usually starts a new main clause with normal main-clause word order:

• men min søster lagde det i lommen ...

This follows the standard Danish main-clause pattern where the finite verb comes in second position:

1. min søster
2. lagde
3. det
4. i lommen på min jakke

So the clause structure is very ordinary here.

Compare:

• Jeg kom hjem, men han blev ude.
• Hun ville gå, men bussen kom ikke.

Not in this exact structure.

The sentence uses the expression:

• var lige ved at glemme = was just about to forget / almost forgot

If you used glemte, you would be saying the forgetting actually happened:

• Jeg glemte kompasset. = I forgot the compass.

But the original sentence means the speaker nearly forgot it, and then the sister prevented that by putting it in the jacket pocket.

So the contrast is:

• Jeg glemte kompasset. = I forgot the compass.
• Jeg var lige ved at glemme kompasset. = I almost forgot the compass.

Yes, it is natural and correct.

It sounds like normal Danish and clearly means:

• my sister put it in the pocket of my jacket

A speaker might also say something slightly different depending on style, for example:

• ... lagde det i jakkelommen
• ... puttede det i lommen på min jakke

But lagde det i lommen på min jakke is perfectly idiomatic and straightforward.