Morska bolest je slabija na mirnom moru.

Literally, morska bolest means sea illness or sea sickness:

• morska = sea / marine
• bolest = illness / sickness

Yes, this is a normal Croatian expression for seasickness. Croatian often uses an adjective + noun combination where English might use a compound noun.

Because morska is an adjective formed from more (sea).

So:

• more = sea
• morski / morska / morsko = sea-, marine

Croatian usually expresses this idea with an adjective, not by putting two nouns together the way English sometimes does. So morska bolest is the natural structure.

Slabija is the comparative form of slaba (weak), but in this sentence it means something like:

• weaker
• less strong
• milder
• less severe

So the sentence means that seasickness is milder / less severe on calm seas.

In context, slabija sounds more natural in English as milder or less severe than literally weaker.

Because slabija has to agree with morska bolest, which is:

• singular
• feminine

Here, slabija is a predicate adjective after je, but it still agrees with the subject.

So:

• bolest is feminine
• therefore the adjective must also be feminine singular: slabija

Good question. Croatian comparatives do not always need an explicit than phrase.

Here, slabija simply means less severe, and the comparison is understood from the context:

• seasickness is milder on calm seas than it is in rougher conditions

If you wanted to make the comparison explicit, Croatian could use nego or od in other sentence types. But in this sentence, the comparison is implicit, which is very natural.

Here na mirnom moru is in the locative case.

After na, Croatian can use:

• accusative for movement toward something
• locative for location or being in a state/place

In this sentence, there is no movement. It means on / in calm sea conditions, so Croatian uses the locative:

• nominative: more
• locative: moru

That is why you see moru.

Because the adjective must agree with the noun in:

• gender
• number
• case

Here:

• more is neuter singular
• after na in this meaning, it is in the locative
• so the adjective must also be neuter singular locative

That gives:

• mirno more = calm sea
• na mirnom moru = on/in a calm sea

So both words change together.

Because Croatian normally says na moru for at sea / on the sea / by the sea, depending on context.

This is one of those prepositional patterns you mostly learn as a set phrase:

• na moru = at sea / by the sea
• u moru = in the sea, inside the water

So in this sentence, na mirnom moru is the natural choice. Using u mirnom moru would suggest being physically in the sea, which is not the intended meaning.

Je is the 3rd person singular of biti (to be): is.

The sentence structure is:

• Morska bolest = subject
• je = is
• slabija = milder / less severe
• na mirnom moru = on calm seas

This is a very normal Croatian word order. Croatian word order is fairly flexible, but this version is neutral and natural.

Yes, to some extent. Croatian word order is more flexible than English because case endings show the grammatical relationships.

For example, a speaker could change the order for emphasis. But the original sentence is the most neutral and straightforward version for a learner:

• Morska bolest je slabija na mirnom moru.

If you move things around, the meaning stays similar, but the emphasis changes.

Croatian does not have articles like English a or the.

So:

• morska bolest can mean seasickness, the seasickness, or a sea-related sickness, depending on context
• na mirnom moru can mean on calm seas or on the calm sea, depending on the situation

The context tells you whether the meaning is general or specific.

Both are possible, but milder is usually the better English translation in this sentence.

• literal: weaker
• natural English in context: milder / less severe

When talking about symptoms, pain, illness, or discomfort, English often prefers mild / milder or less severe, even if Croatian uses a form based on slab (weak).

Usually it means calm sea conditions in general.

So the sentence is best understood as:

• seasickness is milder when the sea is calm

Because Croatian has no articles, the phrase does not force a distinction between a calm sea and the calm sea. Context decides that, and here the general meaning is the most likely one.