Jag måste gå tillbaka till stationen, eftersom jag glömde min väska där.

Måste is the present tense of måste, the modal verb meaning must / have to.

In this sentence, Jag måste gå tillbaka ... means I have to go back ... or I must go back .... In everyday English, have to often sounds more natural, but Swedish måste covers both ideas well.

A few useful points:

• jag måste = I must / I have to
• du måste = you must / you have to
• vi måste = we must / we have to

After måste, the next verb stays in the infinitive:

• Jag måste gå = I must go
• Hon måste läsa = She has to read

So måste gå tillbaka literally means must go back.

Because gå tillbaka means go back / return, while gå by itself just means go or walk.

So:

• gå = go, walk
• gå tillbaka = go back
• komma tillbaka = come back

In your sentence, the speaker is returning to a place they were at before, so tillbaka is important.

Examples:

• Jag går till stationen. = I’m going to the station.
• Jag går tillbaka till stationen. = I’m going back to the station.

Without tillbaka, the idea of back disappears.

Because stationen means the station.

In Swedish, the definite article is usually added to the end of the noun:

• en station = a station
• stationen = the station

So:

• till en station = to a station
• till stationen = to the station

This is very common in Swedish. English uses a separate word (the), but Swedish often uses an ending instead.

In this sentence, the speaker means a specific station, so stationen is natural.

Because station is a common-gender noun in Swedish, so it takes en in the indefinite form.

Its forms are:

• en station = a station
• stationen = the station

This also affects possessives and adjectives in other contexts, though here the main thing to notice is the definite ending -en.

Many Swedish nouns are either:

• en words
• ett words

You usually have to learn the noun together with its gender:

• en station
• en väska
• ett hus

Because eftersom introduces a subordinate clause, and Swedish word order changes in subordinate clauses.

Main clause word order is usually:

• subject + verb
• Jag måste gå tillbaka ...

Subordinate clause word order is also subject + verb, but unlike main clauses, you do not do verb-second inversion after a subordinating word like eftersom.

So you say:

• eftersom jag glömde min väska där

not:

• eftersom glömde jag min väska där

Compare:

• Jag glömde min väska där. = main clause
• ... eftersom jag glömde min väska där. = subordinate clause

This is an important pattern in Swedish.

Because the forgetting happened earlier, even though the need to go back is happening now.

The sentence has two time layers:

• Jag måste gå tillbaka ... = what is true now
• ... eftersom jag glömde min väska där. = what happened before

So glömde is past tense because the speaker already forgot the bag.

This is very natural in both Swedish and English:

• I have to go back because I forgot my bag there.
• Jag måste gå tillbaka eftersom jag glömde min väska där.

Yes, you often can.

• Jag glömde min väska där. = I forgot my bag there.
• Jag har glömt min väska där. = I have forgotten my bag there.

Both can work, but the nuance is slightly different:

• glömde focuses more on the past event
• har glömt often emphasizes the present result: the bag is still left there

So:

• Jag måste gå tillbaka till stationen, eftersom jag glömde min väska där.
• Jag måste gå tillbaka till stationen, eftersom jag har glömt min väska där.

Both are understandable and natural. The original version with glömde is completely fine.

Because väska is an en word, so the possessive form is min.

Compare:

• en väska → min väska = my bag
• ett hus → mitt hus = my house

So the possessives match the gender of the noun:

• min for many en words
• mitt for ett words
• mina for plural

Examples:

• min bok = my book
• mitt äpple = my apple
• mina skor = my shoes

Since väska is en väska, the correct form is min väska.

Där means there.

In this sentence, it refers back to stationen:

• ... jag glömde min väska där.
• ... I forgot my bag there.

Putting där at the end is very natural in Swedish, just like in English.

The idea is:

• på stationen could mean at the station
• but där avoids repeating stationen

So instead of saying:

• ... eftersom jag glömde min väska på stationen

the speaker says:

• ... eftersom jag glömde min väska där

Both are possible, but där sounds smooth and natural because the place was already mentioned.

Yes. Swedish has a few ways to say because.

Common options include:

• eftersom = because / since
• därför att = because
• sometimes för att, though that can also mean in order to, so it can be less clear for learners

Your sentence could also be:

• Jag måste gå tillbaka till stationen, därför att jag glömde min väska där.

That is correct, but eftersom is very natural here.

For learners, eftersom is a very safe and useful choice when giving a reason.

In this sentence, the comma is natural and standard, especially because the sentence is fairly long and the eftersom clause comes after the main clause.

• Jag måste gå tillbaka till stationen, eftersom jag glömde min väska där.

In modern Swedish, commas are often used a bit less strictly than in English, and you may sometimes see the sentence without a comma:

• Jag måste gå tillbaka till stationen eftersom jag glömde min väska där.

That version is also commonly accepted.

So the comma is not the most important thing for understanding. What matters more is recognizing that eftersom introduces the reason.

Because Swedish modal verbs do not use a separate word like English to before the next verb.

In English:

• I have to go
• I want to go

In Swedish:

• Jag måste gå
• Jag vill gå

After verbs like måste, kan, vill, ska, the next verb usually appears in the infinitive without any extra word.

Examples:

• Jag kan simma. = I can swim.
• Hon vill läsa. = She wants to read.
• Vi måste åka. = We have to leave/go.

So måste gå is exactly what you should expect.

Yes, depending on how the person is traveling.

• gå tillbaka = go back on foot / walk back
• åka tillbaka = go back by vehicle, or more generally travel back

In everyday Swedish, gå tillbaka can sometimes still be used a bit generally, but its basic meaning is physical going/walking.

So if the person is literally walking back to the station, gå tillbaka is perfect. If they are taking a train, bus, or car back, åka tillbaka may be more precise.

Examples:

• Jag måste gå tillbaka till stationen. = I have to walk/go back to the station.
• Jag måste åka tillbaka till stationen. = I have to travel back to the station.

The original sentence is already natural, but in casual speech, people might shorten or simplify it a little.

For example:

• Jag måste tillbaka till stationen, jag glömde min väska där.
• Jag måste gå tillbaka till stationen för jag glömde min väska där.

A few notes:

• Dropping gå can happen when tillbaka already makes the movement clear.
• för is common in speech as a shorter casual way of saying because, though learners should first get comfortable with eftersom.

So the original sentence is a very good, clear standard version, while spoken Swedish may be slightly looser.