Hon sa att vägen kan vara farlig när dimman ligger över den och bilarna kör fort.

Both sa and sade are accepted past-tense forms of säga (to say).

• Hon sa = She said
• Hon sade = also She said

Sa is very common in everyday modern Swedish and often sounds more natural in speech. Sade can sound a bit more formal, careful, or old-fashioned depending on context.

Att means that in this sentence:

• Hon sa att ... = She said that ...

It introduces a subordinate clause. In spoken Swedish, att can sometimes be left out after verbs like säga, just as that can be omitted in English:

• Hon sa att vägen kan vara farlig
• Hon sa vägen kan vara farlig

Both are possible, but keeping att is very standard and clear, especially in writing.

Väg means road.
Vägen means the road.

The ending -en is the definite article attached to the noun:

• en väg = a road
• vägen = the road

Swedish usually adds the definite article to the end of the noun instead of putting a separate word like the in front.

Kan vara means can be or may be, so it expresses possibility rather than certainty.

• vägen är farlig = the road is dangerous
  This sounds more definite.
• vägen kan vara farlig = the road can be dangerous / may be dangerous
  This means it is dangerous under certain conditions or possibly dangerous.

In this sentence, the speaker is not saying the road is always dangerous, only that it can be dangerous when the fog is over it and cars are driving fast.

Because farlig agrees with vägen, which is a singular en-word.

Adjectives used after vara still agree with the noun:

• en väg → vägen är farlig
• ett hus → huset är farligt
• vägarna är farliga → the roads are dangerous

So here:

• vägen = singular, common gender
• therefore farlig

När means when.

In this sentence, it introduces the situation in which the road can be dangerous:

• när dimman ligger över den och bilarna kör fort
• when the fog lies over it and the cars drive fast

A learner might wonder about om, but om means if, not when.
Here the meaning is about a time/situation that happens, so när is the correct choice.

Literally, dimman ligger över den means the fog lies over it.

Yes, that is natural Swedish. Swedish often uses ligga (lie / be situated / rest) for things like fog, snow, clouds, or coverings spread over something.

So:

• dimman ligger över vägen = the fog is over the road
• dimman ligger över den = the fog is over it

In English, lies over sounds a bit literary, but in Swedish ligger över is a normal way to describe this.

Because den refers back to vägen, and väg is an en-word.

Swedish pronouns match the gender of the noun:

• en väg → den
• ett hus → det

So:

• vägen → den

That is why the sentence says över den (over it).

Yes. You could also say:

• när dimman ligger över vägen

That means exactly when the fog lies over the road.

Using den avoids repetition, just like it in English. Both are natural, but den is very common because the noun has already been mentioned.

Bilarna means the cars, while bilar means cars.

• bilar = indefinite plural
• bilarna = definite plural

In Swedish, definite forms are often used where English might use a more general noun phrase. Here bilarna kör fort can mean the cars are driving fast, but depending on context it may also sound like cars are driving fast there / the cars on that road are driving fast.

So this definite form is natural Swedish even if English might sometimes use a less definite wording.

Both fort and snabbt can relate to speed, but fort is especially common as an adverb meaning fast / quickly with movement:

• köra fort = drive fast
• springa fort = run fast

Snabbt is also possible in many cases, but fort is often the more idiomatic choice with driving and movement.

So:

• bilarna kör fort sounds very natural.

The sentence contains one main clause and two subordinate clauses:

1. Hon sa ...
   Main clause: She said ...

2. att vägen kan vara farlig
   Subordinate clause introduced by att

3. när dimman ligger över den och bilarna kör fort
   Subordinate clause introduced by när, with two coordinated parts:

   • dimman ligger över den
   • bilarna kör fort

Basic word order here is straightforward:

• Hon sa
• att vägen kan vara farlig
• när dimman ligger över den
• och bilarna kör fort

A useful thing to know is that Swedish main clauses normally follow V2 word order (the finite verb is in second position), but subordinate clauses do not follow that pattern in the same way. This sentence is a good example of normal subordinate-clause order.

Yes. Hon means she, and Swedish does not capitalize pronouns just because they refer to a person.

So:

• Hon sa ... because it begins the sentence
• in the middle of a sentence it would be hon

This is the same as in modern English with she, but unlike German, for example, where pronoun capitalization rules are different.

Yes, that is a very reasonable way to understand the grammar.

Some parts are flexible in English:

• kan vara = can be / may be
• dimman ligger över den = the fog is over it / fog lies over it / fog covers it
• bilarna kör fort = the cars drive fast / the cars are driving fast / cars are going fast

So even if the exact English wording changes, the Swedish structure stays the same.