Det var en hemsk dag.

In this sentence, det means it.

So Det var en hemsk dag literally matches English It was a terrible day.

Swedish often uses det in the same kind of general way English uses it, especially in statements like:

• Det var roligt. = It was fun.
• Det var sent. = It was late.
• Det var en hemsk dag. = It was a terrible day.

You do not need det to match dag in gender here. It is just the normal subject it.

Var is the past tense of vara, which means to be.

So:

• är = is / am / are (present)
• var = was / were (past)
• vara = to be

Examples:

• Det är en bra dag. = It is a good day.
• Det var en bra dag. = It was a good day.

So in your sentence, var tells you that the sentence is in the past.

Because dag is a common gender noun in Swedish, so it takes en.

Swedish nouns have two grammatical genders:

• en-words (common gender)
• ett-words (neuter)

dag is an en-word, so you say:

• en dag = a day

not:

• ett dag

This is something you usually have to learn together with each noun.

The adjective changes form depending on the noun it describes.

Here, dag is:

• singular
• indefinite
• an en-word

For that combination, the basic adjective form is used:

• en hemsk dag

Compare:

• ett hemskt hus = a terrible house
• en hemsk dag = a terrible day
• hemska dagar = terrible days
• den hemska dagen = the terrible day

So hemsk is correct because it matches an en-word in the singular indefinite form.

En here is the indefinite article, meaning a or an.

So:

• en dag = a day

In Swedish, indefinite articles depend on noun gender:

• en for common gender nouns
• ett for neuter nouns

So in this sentence:

• en hemsk dag = a terrible day

Because Swedish places the article before the whole noun phrase, just as English does:

• en hemsk dag

Here:

• en = a
• hemsk = terrible
• dag = day

So en already covers the whole phrase hemsk dag. You do not add another article.

The basic word order is:

subject + verb + complement

So:

• Det = subject
• var = verb
• en hemsk dag = complement

This is very similar to English:

• It
  • was
    • a terrible day

Swedish and English often look quite similar in simple statements like this.

Yes, you can say Dagen var hemsk, but it means something slightly different in structure.

• Det var en hemsk dag. = It was a terrible day.
• Dagen var hemsk. = The day was terrible.

The first sentence presents a terrible day as a description of what it was. The second makes the day the subject and comments on it directly.

Both are natural, but they are not exactly the same sentence.

Because in Swedish, singular countable nouns usually need an article when they are indefinite.

So you say:

• en dag = a day

not usually:

• dag

Since dag is a singular countable noun, en is needed here:

• Det var en hemsk dag.

This works much like English, where you also need a in It was a terrible day.

Hemsk usually means terrible, awful, or horrible, and it is quite a strong word.

So en hemsk dag suggests a very bad day, not just a slightly unpleasant one.

Depending on context, Swedish also uses milder adjectives like:

• dålig = bad
• jobbig = tough / annoying / exhausting

So:

• en dålig dag = a bad day
• en jobbig dag = a rough/tiring day
• en hemsk dag = a terrible day

A simple learner-friendly approximation would be:

deh var en hemsk dahg

A few notes:

• det is often pronounced more like de or deh in normal speech
• var sounds roughly like vahr
• en is often pronounced lightly, sometimes almost like ehn or just n
• hemsk has a short vowel and a sharp final consonant cluster
• dag has a long a sound

Pronunciation varies a bit by region and speaking style, but that approximation will get you close.

Yes. Det var is extremely common in Swedish.

You can use it for descriptions, situations, time, weather-like expressions, and general comments. For example:

• Det var kallt. = It was cold.
• Det var svårt. = It was difficult.
• Det var en lång film. = It was a long film.
• Det var mycket folk där. = There were a lot of people there.

So learning det var as a common chunk is very useful.