Ég sef illa þegar ég er stressaður.

Because sofa is the infinitive meaning to sleep, while sef is the 1st person singular present tense form, meaning I sleep.

So:

• að sofa = to sleep
• ég sef = I sleep

This verb changes its stem in the present tense, so it is not fully regular. A few useful forms are:

• ég sef = I sleep
• þú sefur = you sleep
• hann/hún/það sefur = he/she/it sleeps
• við sofum = we sleep
• þið sofið = you sleep
• þeir/þær/þau sofa = they sleep

So Ég sef illa is the correct way to say I sleep badly.

Illa is an adverb, and here it means badly or poorly.

It is used because it modifies the verb sef — it tells you how the sleeping happens.

• Ég sef illa = I sleep badly / I sleep poorly

Compare that with an adjective, which would describe a noun instead.

A useful connection is:

• illur = bad, ill, evil (adjective)
• illa = badly, poorly (adverb)

English also does this:

• bad → adjective
• badly → adverb

So Icelandic uses illa because you are describing the action of sleeping, not a noun.

Here þegar means when.

It introduces the time clause:

• þegar ég er stressaður = when I am stressed

In a sentence like this, it often has the sense of whenever as well, especially if you are talking about a repeated pattern:

• Ég sef illa þegar ég er stressaður. = I sleep badly when I am stressed. = I tend to sleep badly whenever I am stressed.

So even though the basic meaning is when, the overall sense can be habitual.

Yes. Stressaður is an adjective, and Icelandic adjectives usually agree with the person they describe in gender, number, and case.

In this sentence, stressaður describes ég, so the form depends on the speaker.

For a male speaker:

• Ég er stressaður.

For a female speaker:

• Ég er stressuð.

So the full sentence would be:

• Ég sef illa þegar ég er stressaður. — said by a man
• Ég sef illa þegar ég er stressuð. — said by a woman

This is a very common thing in Icelandic, so learners need to get used to adjectives changing form like this.

Because Icelandic, like English, normally needs a subject in each clause.

This sentence has two clauses:

1. Ég sef illa = I sleep badly
2. þegar ég er stressaður = when I am stressed

Each clause has its own verb:

• sef = sleep
• er = am

And each clause also needs its own subject:

• ég = I
• ég = I

So even though it sounds repetitive to an English speaker, it is completely normal and necessary.

Yes, absolutely.

You can say:

• Þegar ég er stressaður, sef ég illa.

This means the same thing.

But notice the word order in the main clause:

• sef ég illa
• not ég sef illa

That happens because Icelandic is a verb-second language in main clauses. If something else comes first — here the whole þegar clause — then the finite verb usually comes before the subject in the main clause.

So:

• Ég sef illa þegar ég er stressaður.
• Þegar ég er stressaður, sef ég illa.

Both are correct, but the second one shows an important Icelandic word-order pattern.

Usually it sounds like a general pattern or habitual truth:

• I sleep badly when I’m stressed.

That is, whenever that situation happens, this is the result.

The present tense in Icelandic often works like English present tense in sentences like this. It can describe:

• a general fact
• a repeated experience
• a usual reaction

If you want to make it clearly about right now, you would usually add another word or rely on context, for example:

• Ég sef illa núna því ég er stressaður. = I’m sleeping badly now because I’m stressed.

So the original sentence most naturally describes a recurring situation.

Yes, it is a normal and very common modern Icelandic word.

It is clearly based on English stress, but it has been adapted to Icelandic spelling and grammar:

• stress
• stressaður
• stressuð
• stressað

That kind of borrowing is common in everyday speech.

You may also hear other ways to express similar ideas, depending on style:

• vera í stressi = to be stressed / be under stress
• vera undir álagi = be under pressure, be under strain

But stressaður is very common and natural in everyday Icelandic.

A rough learner-friendly pronunciation is:

yeg sev IT-la THEH-gar yeg er STRES-sa-thur

A few helpful notes:

• Ég begins with a y sound: roughly yeg
• sef is pronounced more like sev than English sef
• illa is not pronounced like English illa; the ll has a special Icelandic sound, often approximated for learners as it-la
• þ is like th in thing
• stressaður has ð, which is like th in this, though in fast speech it can be quite soft

Also, Icelandic usually stresses the first syllable of each word:

• ÉG
• SEF
• IL-la
• ÞE-gar
• ÉG
• ER
• STRES-sa-ður

If you want to sound more natural, getting the rhythm and first-syllable stress right helps a lot.