Высота волны усиливает морскую болезнь.

Because волны here is genitive singular, not nominative.

In Russian, when one noun depends on another like height of the wave, the second noun is usually put in the genitive:

• высота = height
• волна = wave
• высота волны = the height of the wave / wave height

So:

• волна = wave
• волны = of the wave

This is a very common pattern in Russian:

• дверь дома = the door of the house
• цвет машины = the color of the car
• высота волны = the height of the wave

Also note that волны could also be nominative plural in other contexts, but here the meaning and structure show that it is genitive singular.

Because the subject of the sentence is высота, not волны.

Russian verbs agree with the subject, and the subject here is:

• высота = height

This is a singular noun, so the verb is singular:

• высота ... усиливает = the height ... increases / worsens

The word волны is only part of the phrase высота волны and is not the main subject by itself.

So the structure is:

• [Высота волны] = subject
• усиливает = verb
• морскую болезнь = object

Because морскую is the accusative singular feminine form of the adjective морской.

The noun it describes is:

• болезнь = illness / sickness

Болезнь is a feminine singular noun, and here it is the direct object of the verb усиливает, so the adjective must match it in gender, number, and case.

Forms of морской:

• masculine nominative: морской
• feminine nominative: морская
• feminine accusative: морскую

So:

• морская болезнь = sea sickness / seasickness
• усиливает морскую болезнь = worsens seasickness

It is in the accusative — it just happens to look the same as the nominative.

The noun болезнь is a feminine noun ending in -ь. For many feminine nouns of this type, the nominative singular and accusative singular are identical:

• nominative: болезнь
• accusative: болезнь

So in this sentence:

• морскую болезнь is accusative
• the adjective shows the case more clearly than the noun does

This is very common in Russian. Sometimes the noun changes visibly in a case, and sometimes it does not.

Усиливает comes from усиливать, an imperfective verb meaning:

• to intensify
• to strengthen
• to increase
• to make worse (depending on context)

In this sentence, the most natural English idea is something like:

• Wave height worsens seasickness
• The height of the wave increases seasickness

Since морская болезнь is something unpleasant, усиливает here has the sense of making it stronger or worse.

Because Russian often uses the present tense for general truths, scientific statements, and habitual facts, just like English does:

• Water boils at 100 degrees.
• Вода кипит при 100 градусах.

So:

• Высота волны усиливает морскую болезнь.

means a general fact:

• Wave height increases/worsens seasickness.

This is normal Russian usage.

Усиливает is the imperfective present form of усиливать.

Aspect pair:

• усиливать = imperfective
• усилить = perfective

The imperfective is used here because the sentence expresses a general relationship or ongoing effect, not one completed event.

So:

• усиливает = increases / worsens / intensifies in general
• усилит would mean will intensify once, as a completed future event

For a general statement, imperfective is the natural choice.

Russian has no articles like a or the. The exact meaning comes from context.

So высота волны can be understood as:

• the height of the wave
• wave height

In a sentence like this, English often prefers the more general phrasing:

• Wave height worsens seasickness

Russian leaves that distinction to context. This is very normal and something English speakers have to get used to.

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

Neutral order here is:

• Высота волны усиливает морскую болезнь.

But you could also say:

• Морскую болезнь усиливает высота волны.

That puts more emphasis on морскую болезнь.

Even though the word order changes, the cases still show:

• высота волны = subject
• морскую болезнь = object

So the meaning stays basically the same, though the emphasis changes.

Yes. Морская болезнь is the normal standard expression for seasickness.

Literally it means:

• морская = sea-related / marine
• болезнь = illness / sickness

So literally it is something like sea sickness, which matches English quite closely.

Not automatically — but in this case, yes, болезнь is feminine.

Nouns ending in -ь can be either masculine or feminine, so you often need to learn their gender individually. For example:

• день = masculine
• словарь = masculine
• болезнь = feminine
• ночь = feminine

You can tell болезнь is feminine because:

• its dictionary gender is feminine
• the adjective agrees with it: морская болезнь

The stress is:

• высота́
• волны́
• усилива́ет
• морску́ю
• боле́знь

So the whole sentence is pronounced approximately as:

высота́ волны́ усилива́ет морску́ю боле́знь

Stress matters a lot in Russian, because it is not always predictable and can affect vowel pronunciation.

The sentence is grammatical and understandable. It sounds somewhat formal or technical, like something from a medical, scientific, or informational text.

A more natural English translation would probably be:

• Wave height worsens seasickness.

In Russian, depending on context, people might also use more conversational wording, but this sentence itself is perfectly fine, especially in explanatory or formal contexts.