Na autocesti se moja majka osjeća sigurnije kad navigacija govori mirno i jasno.

Because Croatian uses na for being on roads, streets, highways, and other surfaces or routes.

• autocesta = highway / motorway
• na autocesti = on the highway

After na in a location meaning, Croatian uses the locative case.
So autocesta changes to autocesti.

This is a very common pattern:

• na cesti = on the road
• na ulici = on the street
• na autocesti = on the highway

Using u autocesti would sound wrong here, because a highway is not treated as an enclosed space.

The verb here is osjećati se, which means to feel in the sense of to feel a certain way.

• osjećati = to feel, sense, perceive something
• osjećati se = to feel, to feel oneself, to be in some state

So:

• Osjećam bol. = I feel pain.
• Osjećam se dobro. = I feel good.

As for word order: se is a clitic, and Croatian clitics usually go very early in the sentence, often in the second position.

Here the first unit is Na autocesti, so the clitic comes right after it:

• Na autocesti se moja majka osjeća...

That is why you do not see moja majka se here, even though English would naturally put the subject first.

After osjećati se, Croatian very often uses an adverb-like form to describe the state someone is in:

• osjećati se dobro = to feel good
• osjećati se loše = to feel bad
• osjećati se sigurno = to feel safe
• osjećati se sigurnije = to feel safer

So sigurnije is the natural form here.

A learner might expect sigurnija because majka is feminine, but that is not the usual structure in this pattern. Croatian normally says:

• Moja majka se osjeća sigurnije.

You may also meet forms like sigurnom / sigurnijom in some constructions, but sigurno / sigurnije is very common and natural with osjećati se.

That is normal. Croatian, like English, often uses the comparative without stating the full comparison if the context makes it clear.

So osjeća se sigurnije means:

• she feels safer
• safer than otherwise
• safer than when the navigation does not speak calmly and clearly

English does the same:

• I feel better when it’s quiet.
• Better than what? Better than when it isn’t quiet.

The comparison is understood from the situation.

Both kad and kada mean when.

• kad = shorter, very common in everyday speech
• kada = a bit fuller, sometimes slightly more formal or emphatic

So:

• kad navigacija govori...
• kada navigacija govori...

Both are correct here.

But ako would mean if, not when, so it would change the meaning.

Because these words describe how the navigation speaks, not what the navigation is like.

So they must be adverbs:

• mirno = calmly
• jasno = clearly

They modify the verb govori:

• navigacija govori mirno i jasno = the navigation speaks calmly and clearly

If you used mirna i jasna, those would be adjectives, describing a feminine noun:

• navigacija je mirna i jasna = the navigation is calm and clear

That is a different structure and a different meaning.

Croatian often uses the present tense for general truths, habits, or repeated situations.

So this sentence is not necessarily about one specific moment right now. It means something like:

• whenever the navigation speaks calmly and clearly, my mother feels safer on the highway

This is a habitual or general statement, so present tense is exactly what Croatian would normally use.

If you were talking about one past situation, you would use the past tense instead:

• Na autocesti se moja majka osjećala sigurnije kad je navigacija govorila mirno i jasno.

Yes. In everyday Croatian, navigacija often means the navigation system, especially GPS or satnav, and by extension even its voice.

So navigacija govori is a natural way to say:

• the GPS speaks
• the satnav gives spoken directions

Croatian often lets the device stand for the voice coming from it, just as English does.

Yes, Croatian word order is fairly flexible, but not completely free.

This sentence could also appear as:

• Moja se majka na autocesti osjeća sigurnije kad navigacija govori mirno i jasno.
• Na autocesti moja se majka osjeća sigurnije kad navigacija govori mirno i jasno.

These versions are all possible, but the placement of the clitic se still has to follow Croatian clitic rules, so it usually stays very early in the clause.

Changing the word order changes the focus slightly:

• Na autocesti... puts focus on the setting
• Moja majka... puts focus on the person

Because Croatian has no articles.

English needs words like:

• the highway
• a highway
• the navigation

Croatian does not have direct equivalents of a/an/the.
So autocesta, majka, and navigacija appear without articles, and the exact meaning is understood from context.

That is why:

• na autocesti can mean on the highway or on a highway
• navigacija can mean the navigation system or just navigation, depending on context