Signalen försvann i en sekund.

Because signalen is the definite singular form: the signal.

• en signal = a signal
• signalen = the signal

In Swedish, the definite article is usually added as an ending, rather than written as a separate word like English the.

Försvann is the past tense of försvinna, which means to disappear or to vanish.

A few useful forms are:

• försvinna = infinitive, to disappear
• försvinner = present, disappears / is disappearing
• försvann = past, disappeared
• försvunnit = supine/past participle-related form, used in perfect constructions, for example har försvunnit = has disappeared

So Signalen försvann means The signal disappeared.

No. In this verb, försvinna is treated as a single verb, and för- is not something you separate in normal use.

So you say:

• Signalen försvann
• Signalen har försvunnit

You do not split it apart the way some particle verbs are split.

I en sekund means for a second / for one second in the sense of during that length of time.

In Swedish, i + time expression is very commonly used to show duration:

• i en sekund = for a second
• i en minut = for a minute
• i en timme = for an hour

So the sentence says that the signal disappeared and stayed gone for that brief period.

Usually, i en sekund is the more natural choice here.

The difference is roughly:

• i en sekund = for one second, describing duration
• under en sekund = in under a second / within one second in some contexts, or literally during one second, but it often feels less natural for this sentence

If you want to say the signal was gone for that amount of time, i en sekund is the normal phrasing.

Because sekund is an en-word in Swedish:

• en sekund
• sekunden
• sekunder
• sekunderna

Swedish nouns belong to two main genders/common classes in basic grammar teaching:

• en-words
• ett-words

Sekund happens to be an en-word.

This is the normal Swedish word order for a simple main clause:

subject + verb + time expression

So:

• Signalen = subject
• försvann = verb
• i en sekund = time/duration phrase

This is very similar to normal English word order: The signal disappeared for a second.

Yes, you can. That is also grammatical.

When a different element comes first in a Swedish main clause, the finite verb usually moves to the second position. So:

• Signalen försvann i en sekund.
• I en sekund försvann signalen.

The second version puts more focus on the time phrase for a second.

This is an example of Swedish V2 word order: the verb stays in the second position of a main clause.

Not always. Very often it is used approximately, just like English for a second.

It can mean:

• literally one second, or
• more loosely, briefly

So depending on context, it may simply mean the signal disappeared momentarily.

Yes, signalen is perfectly natural, especially in contexts involving:

• internet connection
• mobile reception
• radio transmission
• electronics
• technical systems

Depending on context, Swedish might also use more specific words, such as:

• mottagningen = the reception
• uppkopplingen = the connection
• nätet = the network
• signalen = the signal

But the sentence itself is completely natural.

A rough guide for English speakers is:

försvann ≈ fur-SVAHN

A few pronunciation notes:

• ö is not the same as English o. It is a rounded front vowel.
• The stress is mainly on svann.
• The rs combination in Swedish often affects the sound quality depending on dialect, but in careful learning pronunciation, saying something close to fur-svahn is a good start.

You do not need perfect pronunciation immediately; the important part is recognizing it as the past tense of försvinna.

Sometimes, yes, depending on context.

Försvinna often means:

• disappear
• vanish
• go missing

So in some situations, it can feel close to was lost or went missing. But in this sentence, with signalen, the most natural sense is disappeared or cut out briefly.

Yes. Depending on context, natural English equivalents could be:

• The signal disappeared for a second.
• The signal cut out for a second.
• We lost the signal for a second.
• The signal dropped for a second.

The best translation depends on whether you are talking about a phone signal, TV signal, internet connection, or something else. In Swedish, though, Signalen försvann i en sekund is a straightforward and natural sentence.