Tiner kapağı kapalı tutuluyor.

Because this is a noun compound in Turkish.

• tiner = thinner / solvent
• kapak = lid, cap

When one noun modifies another in this kind of structure, the second noun usually takes the 3rd person possessive suffix:

• kapak → kapağı

So:

• tiner kapağı = the thinner lid / the lid of the thinner

Also, the final k in kapak softens to ğ before a vowel-initial suffix, which is why you get kapağı, not kapakı.

Both forms are possible, but they are used a bit differently.

• tiner kapağı = an indefinite noun compound, very common for naming things
• tinerin kapağı = explicitly the lid of the thinner

In many everyday contexts, Turkish prefers the shorter compound form:

• tiner kapağı

If you say:

• tinerin kapağı

it sounds more explicitly possessive, as if you are referring to the lid of a specific thinner container.

So in a label, instruction, or general statement, tiner kapağı is very natural.

Kapalı is an adjective meaning closed.

Here it describes the state of the lid:

• kapalı = closed
• açık = open

So kapalı tutuluyor means something like:

• is being kept closed
• is kept closed

It describes a state that is maintained, not the action of closing itself.

Because kapalı tutuluyor and kapatılıyor mean different things.

• kapatılıyor = it is being closed
  This focuses on the action of closing.
• kapalı tutuluyor = it is being kept closed
  This focuses on the resulting state being maintained.

So:

• Tiner kapağı kapatılıyor = someone is closing the thinner lid
• Tiner kapağı kapalı tutuluyor = the thinner lid is kept in a closed state

In English, this difference is similar to:

• is being closed vs.
• is kept closed

It comes from the verb tutmak, which has meanings like to hold, to keep, to maintain.

Step by step:

• tutmak = to hold / keep
• tutulmak = to be held / to be kept
  (-ul/-ıl here makes it passive)
• tutuluyor = is being held / is being kept
  (-yor is the present continuous marker)

So:

• kapalı tutuluyor literally means is being kept closed

Yes.

The active verb would be:

• birisi kapağı kapalı tutuyor = someone is keeping the lid closed

The sentence you have is passive:

• kapak kapalı tutuluyor = the lid is being kept closed

The passive lets Turkish avoid mentioning who is doing it, which is very common in instructions, warnings, and general statements.

Yes, -yor is the present continuous form, but in Turkish it is used more broadly than English is ...ing.

It can express:

• an action happening now
• a current ongoing situation
• a general practice or policy
• something regularly done in the present context

So kapalı tutuluyor does not have to mean that someone is physically keeping it closed at this exact second. It can also mean:

• it is kept closed
• it’s normally kept closed
• it’s being kept closed

This is very natural Turkish.

The subject is:

• Tiner kapağı = the thinner lid / the lid of the thinner

The rest of the sentence tells you what happens to it:

• kapalı tutuluyor = is being kept closed

So the structure is:

• [Subject] + [state/complement] + [verb]

That is very common in Turkish.

Because kapalı functions as a complement describing the state in which the lid is kept.

Turkish often puts this kind of descriptive word before the verb:

• açık bırakmak = to leave open
• kapalı tutmak = to keep closed
• hazır bulundurmak = to keep ready

So:

• kapalı tutuluyor = is kept closed

This is a fixed and very natural pattern.

Not exactly.

If you simply want to say The lid is closed, Turkish would more naturally say:

• Tiner kapağı kapalı.

That just describes the current state.

But your sentence is:

• Tiner kapağı kapalı tutuluyor.

This adds the idea of maintenance or control:

• it is kept closed
• people make sure it stays closed

So the sentence is stronger than just a plain description.

No. Turkish does not have articles like English a/an/the.

So whether you translate it as:

• the thinner lid
• a thinner lid
• the lid of the thinner

depends on context.

In this sentence, English usually uses the because the lid is understood as a specific, known part of the container.

Because Turkish often uses the passive when the doer is:

• unknown
• unimportant
• obvious from context
• intentionally left unstated

This is especially common in:

• instructions
• technical descriptions
• warnings
• formal writing

So kapalı tutuluyor is a very natural way to say this without naming the person or people responsible.

Yes, very natural.

It has the kind of impersonal, instruction-like tone often used in Turkish for labels and safety statements.

Depending on context, you might also see:

• Tiner kapağı kapalı tutulmalıdır. = The thinner lid should be kept closed.
• Tinerin kapağı kapalı tutulmalıdır.
• Kapağı kapalı tutunuz. = Keep the lid closed.

Your sentence sounds like a neutral statement of practice or condition:

• The thinner lid is kept closed.

They express different meanings:

• kapalı tutuluyor = is being kept closed / is kept closed
• kapalı tutulmalı = should be kept closed

So:

• tutuluyor describes what is happening
• tutulmalı gives a recommendation, rule, or obligation

For safety instructions, tutulmalı is often more direct.

Yes. In real usage, that is exactly how many people would understand it.

Turkish often names parts of objects with noun compounds like this:

• şişe kapağı = bottle cap
• kavanoz kapağı = jar lid
• araba kapısı = car door

So tiner kapağı naturally means the lid/cap belonging to the thinner container.

A little, but the given order is the most natural.

Normal order:

• Tiner kapağı kapalı tutuluyor.

You could move words around for emphasis, but Turkish usually keeps the verb at the end, and the standard neutral order is best here.

For example:

• Kapalı tutuluyor tiner kapağı.

is possible in some special contexts, but it sounds marked or poetic, not neutral.

So learners should treat:

• Tiner kapağı kapalı tutuluyor.

as the normal pattern.

You could, and it helps show the grammar, but it is not the best natural English translation in most contexts.

A more natural English version is:

• The thinner lid is kept closed.
• The lid of the thinner is kept closed.

The literal sense of tutmak = hold/keep is still there in Turkish, but in this expression English usually prefers keep closed rather than hold closed.