Deniz tutması başladı.

Deniz tutması is a common expression meaning seasickness.

• deniz = sea
• tutması comes from tutmak = to hold, catch, grip

So the phrase is built from the idea of the sea gripping/catching someone, but in real use you should learn deniz tutması as a single expression meaning seasickness.

Because deniz tutması is a compound noun.

• tutma is a verbal noun formed from tutmak
• -sı / -si / -su / -sü is the usual ending on the second part of many Turkish noun compounds

So here -sı does not mean a literal his/her/its in the way English learners often expect. It is just part of the normal compound pattern.

Compare:

• baş ağrısı = headache
• diş fırçası = toothbrush
• deniz tutması = seasickness

Başladı means started or began.

It is the simple past, 3rd person singular form of başlamak = to start / to begin.

So the sentence is literally:

Deniz tutması başladı.
The seasickness started.

In natural English, depending on context, you might also say:

• I started to feel seasick.
• The seasickness came on.

The subject is the whole phrase deniz tutması.

So the structure is:

• deniz tutması = subject
• başladı = verb

That is why the verb is in 3rd person singular: the subject is seasickness, not I, you, or they.

Turkish often leaves that kind of information to context.

This sentence only says that seasickness started. It does not directly name the person affected. English often prefers to make that person explicit, so in context you might translate it as:

• I started feeling seasick.
• She started feeling seasick.

If Turkish wants to mention the person more clearly, it can do so in other ways, for example:

• Beni deniz tuttu. = I got seasick.

Not in a literal physical sense.

In expressions like this, tutmak can have the idea of:

• to seize
• to come over
• to affect

So the image is something like the sea seized/caught someone, which becomes the idiomatic meaning to get seasick.

That is why it is best to learn deniz tutması or beni deniz tuttu as set expressions, rather than translating tutmak word-for-word every time.

It is capitalized here simply because it is the first word of the sentence.

So here it means sea, not the name Deniz.

In the middle of a sentence, it would normally be written deniz unless it were actually a person’s name.

Turkish does not have an article that works exactly like English the.

So Deniz tutması başladı can naturally mean:

• The seasickness started
• Seasickness started

The exact English wording depends on context.

It is largely a fixed, established expression.

A learner should treat deniz tutması as a normal vocabulary item meaning seasickness. Even though you can analyze its parts grammatically, native speakers know it as a standard expression.

A related common sentence is:

• Beni deniz tuttu. = I got seasick.

So both the noun expression and the verb expression are worth learning as chunks.

Yes. This is a normal, neutral order.

• deniz tutması = subject
• başladı = verb

So Deniz tutması başladı is the straightforward way to say it.

Turkish word order can change for emphasis, but this version sounds plain and natural.