Jeg finder mit rejsekort i jakkelommen, før jeg går om bord.

Because rejsekort is a neuter noun in Danish.

Danish indefinite articles and possessives change with grammatical gender:

• en noun → min
• et noun → mit

So:

• et rejsekort = a travel card
• mit rejsekort = my travel card

If the noun were a common-gender noun, you would use min instead.

Yes, rejsekort is one compound word, which is very common in Danish.

It is made from:

• rejse = travel/journey
• kort = card

So rejsekort literally means travel card. In Denmark, Rejsekort is also the name of the electronic public transport card system, so depending on context it can mean either:

• a general travel card, or
• the specific Danish Rejsekort

Danish often joins words where English uses separate words.

Both are possible, but they mean slightly different things.

• i jakkelommen = in the jacket pocket
• i min jakkelomme = in my jacket pocket

In Danish, when the ownership is obvious from the context, it is very natural to use the definite form without a possessive. So i jakkelommen often sounds more natural than explicitly saying my jacket pocket.

This is similar to how Scandinavian languages often prefer the where English prefers my.

Because -en is the definite singular ending for many common-gender nouns in Danish.

The basic noun is:

• en jakkelomme = a jacket pocket

The definite form is:

• jakkelommen = the jacket pocket

So in the sentence:

• i jakkelommen = in the jacket pocket

Also notice that jakkelomme itself is a compound:

• jakke = jacket
• lomme = pocket

Finder is the present tense of finde.

It can mean:

• I find
• I am finding
• sometimes even a near-future idea, depending on context

In this sentence, it most naturally means something like:

• I find/get hold of my travel card in my jacket pocket before I board
• or I make sure I have my travel card ready in my jacket pocket before boarding

Danish present tense is often used in ways that English might translate with either simple present or present progressive, depending on context.

Før means before.

It introduces a subordinate clause:

• før jeg går om bord = before I board / before I go on board

So the sentence has:

• main clause: Jeg finder mit rejsekort i jakkelommen
• subordinate clause: før jeg går om bord

This works very much like English before.

Yes, gå om bord is a common fixed expression meaning to board.

Literally:

• går = go/walk
• om bord = aboard/on board

Together, gå om bord means board a vehicle, ship, train, bus, plane, etc.

So:

• jeg går om bord = I board / I go on board

In English we usually translate the whole phrase idiomatically as board, not word-for-word.

Because om bord is an established Danish expression, and it is normally written as two words.

Examples:

• at gå om bord = to board
• at være om bord = to be on board

So even though English often uses one word like aboard, Danish keeps om bord separate in this expression.

Because før jeg går om bord is a subordinate clause, and Danish normally uses a comma before subordinate clauses.

So:

• Jeg finder mit rejsekort i jakkelommen, før jeg går om bord.

This comma is standard in Danish writing. English also often uses a comma before before clauses when they come after the main clause, but Danish comma rules are their own system and are generally more formalized in this area.

Yes, but only in a normal Danish way.

In the main clause, Danish follows the V2 rule: the finite verb usually comes in the second position.

• Jeg finder mit rejsekort ...

In the subordinate clause after før, Danish does not use V2 word order. It keeps the subject before the verb in the ordinary subordinate-clause pattern:

• før jeg går om bord

So this is standard grammar:

• jeg finder ... = main clause
• før jeg går ... = subordinate clause

Yes. Inden can also mean before, and in many contexts it is very close to før.

So this would also be natural:

• Jeg finder mit rejsekort i jakkelommen, inden jeg går om bord.

In many everyday situations, før and inden overlap. Learners will often hear both. In this sentence, før sounds perfectly normal and idiomatic.