Морская болезнь беспокоит пассажира.

Because пассажира is in the accusative case: it is the direct object of беспокоит.

A very important point for learners is that with animate masculine nouns in the singular, the accusative form looks like the genitive form.

So:

• пассажир = nominative, the passenger
• пассажира = accusative/genitive form

In this sentence:

• Морская болезнь = the subject
• беспокоит = bothers
• пассажира = the person being bothered

If the noun were inanimate, the accusative would usually look like the nominative instead.

Because Russian grammar shows this through case endings, not mainly through word order.

Here:

• болезнь is in the nominative case, which marks the subject
• пассажира is in the accusative case, which marks the direct object

So the structure is:

• Морская болезнь = seasickness
• беспокоит = bothers
• пассажира = the passenger

Even though English often relies heavily on word order, Russian uses case endings to show who is doing what to whom.

Because the adjective морская must agree with the noun болезнь.

The noun болезнь is:

• feminine
• singular
• nominative

So the adjective must also be:

• feminine
• singular
• nominative

That gives:

• морская = feminine singular nominative
• болезнь = feminine singular nominative

This is standard adjective-noun agreement in Russian.

Yes. Nouns ending in -ь can be either masculine or feminine, so you have to learn their gender individually.

Болезнь is a feminine noun.

That is why you get:

• морская болезнь
• not морской болезнь

This noun belongs to what learners often call the third declension, which includes many feminine nouns ending in -ь.

Because its subject, морская болезнь, is singular.

Russian verbs in the present tense agree with the subject in number and person.

Here the subject is one thing:

• морская болезнь = singular

So the verb is also singular:

• беспокоит = it bothers

If the subject were plural, the verb would also be plural:

• Морские болезни беспокоят пассажира.

Беспокоит is the 3rd person singular present tense form of беспокоить.

So literally it means:

• (it) bothers
• (it) worries
• (it) troubles

In this sentence, the subject is морская болезнь, so беспокоит means seasickness is bothering the passenger.

Here беспокоить means something like:

• to bother
• to trouble
• to cause discomfort to
• sometimes to worry

In this sentence, it does not mean that the passenger is emotionally worrying about seasickness. It means seasickness is causing discomfort to the passenger.

So this is a very natural Russian way to express that something is troubling or bothering someone.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

For example, these are still understandable:

• Пассажира беспокоит морская болезнь.
• Беспокоит пассажира морская болезнь.

But the original order:

• Морская болезнь беспокоит пассажира.

is a neutral, straightforward way to present the information.

Changing the order can shift emphasis:

• Пассажира беспокоит морская болезнь puts more focus on the passenger
• Морская болезнь беспокоит пассажира starts with the cause, seasickness

Russian does not have articles like English the and a/an.

So:

• морская болезнь can mean seasickness, the seasickness, or a case of seasickness
• пассажира can mean the passenger or a passenger

The exact meaning depends on context.

In a simple sentence like this, English usually chooses the passenger because it sounds natural, but Russian itself does not mark that distinction with an article.

A helpful stressed version is:

• морскАя болЕзнь беспокОит пассажИра

Roughly:

• мор- SKA -ya
• ba-LYEZN’
• be-spa- KO -it
• pa-sa- ZHEE -ra

A few useful notes:

• болезнь ends with a soft consonant sound because of ь
• г in пассажира is not there; the ж sound is like the s in measure
• stress matters a lot in Russian, so it is worth memorizing words with stress

No. That would be ungrammatical.

After беспокоить, the person affected is the direct object, so пассажир must be in the accusative:

• correct: пассажира
• not correct here: пассажир

The form пассажир would be nominative, which would suggest it is the subject.

It can be understood either way, depending on context.

The Russian present tense often covers both:

• something happening right now
• a more general or ongoing situation

So this could mean:

• The passenger is being bothered by seasickness
• or Seasickness bothers the passenger

Context tells you which interpretation is intended.