Vittnet såg tjuven utanför huset.

Because Swedish usually puts the definite article on the end of the noun.

• vittne = witness → vittnet = the witness
• tjuv = thief → tjuven = the thief
• hus = house → huset = the house

So where English says the witness, Swedish often says just vittnet.

A useful extra point: if there is an adjective, Swedish normally uses both a separate article and the ending:

• det stora huset = the big house

Because Swedish grammatical gender is not the same as natural gender.

Even though a witness is a person, the noun vittne is grammatically an ett noun:

• ett vittne = a witness
• vittnet = the witness

This is something you usually just have to learn with the noun. Not all person-words are en words.

Såg is the past tense of se, which means to see.

Some forms are:

• se = to see
• ser = sees / is seeing
• såg = saw
• sett = seen

So Vittnet såg tjuven utanför huset is in the simple past: The witness saw the thief outside the house.

In a normal Swedish statement, word order usually tells you that.

Here the pattern is:

• Vittnet = subject
• såg = verb
• tjuven = object

So vittnet is the one who saw, and tjuven is the one who was seen.

Unlike some languages, Swedish nouns usually do not change form for subject and object. That means word order is very important.

Because Swedish main clauses usually follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• Vittnet = first element
• såg = second element

That is exactly what Swedish expects in a normal main clause.

You can move another element to the front, but the verb still stays second:

• Utanför huset såg vittnet tjuven.

That still means roughly the same thing, but now Utanför huset is the first element, so såg must still come second.

Utanför means outside, and utanför huset means outside the house.

In this sentence, it is a place expression. It tells us where the relevant action or person is located.

• utanför = outside
• huset = the house

So together: outside the house

This sentence can be a little ambiguous.

The most natural reading is usually that the thief was outside the house:

• The witness saw the thief [who was] outside the house.

But depending on context, it could also be understood as the place where the seeing happened.

If you want to make the first meaning clearer, Swedish could say something like:

• Vittnet såg tjuven som stod utanför huset.
  • The witness saw the thief who was standing outside the house.

If you want to make the second meaning clearer, you could say:

• Utanför huset såg vittnet tjuven.
  • Outside the house, the witness saw the thief.

You would use the indefinite forms:

• Ett vittne såg en tjuv utanför ett hus.

Compare:

• vittnet / tjuven / huset = the witness / the thief / the house
• ett vittne / en tjuv / ett hus = a witness / a thief / a house

So Swedish uses separate words for the indefinite article (en, ett), but usually a suffix for the definite article.

Put the finite verb first:

• Såg vittnet tjuven utanför huset?

That means:

• Did the witness see the thief outside the house?

This is a very common pattern in Swedish yes/no questions:

• statement: Vittnet såg tjuven utanför huset.
• question: Såg vittnet tjuven utanför huset?

Yes:

• Utanför huset såg vittnet tjuven.

This is grammatical and still means Outside the house, the witness saw the thief.

The important thing is that Swedish keeps the finite verb in second position, so after moving Utanför huset to the front, såg still has to come next.