Altyazı ile film izlemek dili geliştirmek için faydalı sayılır.

It means with subtitles.

• altyazı = subtitle
• ile = with, by means of

So altyazı ile film izlemek means to watch films with subtitles.

In everyday Turkish, ile is often attached to the noun, so you may also see:

• altyazıyla film izlemek

A very common alternative is:

• altyazılı film izlemek = to watch subtitled films

Both are natural, but altyazı ile focuses a bit more on the idea of using subtitles as a tool.

Because here izlemek is functioning like watching in English.

The whole phrase Altyazı ile film izlemek acts as the subject of the sentence:

• Altyazı ile film izlemek = watching films with subtitles

Turkish often uses the -mek / -mak infinitive form this way to turn a verb into a noun-like expression.

So the sentence structure is basically:

• [Watching films with subtitles] [is considered useful]

Because film here is being used in a general sense: watching films, not watching a specific film.

• film izlemek = to watch films / to watch a film in general
• filmi izlemek = to watch the film / a specific film already known

This is similar to English:

• Watching films is useful → general
• Watching the film is useful → specific

Dili is the accusative form of dil, and it marks a definite direct object.

• dil = language
• dili = the language

In this sentence, it refers to the language being learned, which is understood from context.

So:

• dili geliştirmek = to improve the language / to improve one’s language ability

A native English speaker may expect something like language skills, and Turkish can say that too:

• dil becerilerini geliştirmek = to improve language skills

But dili geliştirmek is a common, compact way to express the idea.

It means in order to improve or for improving.

• geliştirmek = to improve, to develop
• için = for, in order to

So:

• dili geliştirmek için = in order to improve the language

This structure is very common in Turkish:

• öğrenmek için = in order to learn
• çalışmak için = in order to work / study
• başarılı olmak için = in order to be successful

Faydalı sayılır means is considered useful or is regarded as useful.

• faydalı = useful, beneficial
• sayılmak = to be considered, to be regarded as

So this is a little softer and more impersonal than faydalıdır.

Compare:

• faydalıdır = it is useful
• faydalı sayılır = it is considered useful

The version with sayılır sounds more like a general opinion or accepted view, rather than a direct assertion.

It comes from the verb saymak, which can mean:

• to count
• to consider
• to regard as

Here it has the consider/regard as meaning.

Then it becomes passive:

• saymak = to consider
• sayılmak = to be considered

And sayılır is the aorist form, often used for general truths, typical situations, or broadly accepted statements:

• faydalı sayılır = is generally considered useful

Because Turkish normally puts the main verb or predicate at the end.

So the sentence follows a very typical Turkish order:

• Altyazı ile film izlemek = subject
• dili geliştirmek için = purpose phrase
• faydalı sayılır = predicate

English is less strict about this, but Turkish strongly prefers the final position for the main verb or verbal predicate.

Both are natural, but Altyazılı film izlemek is often a bit more idiomatic in everyday Turkish.

Compare:

• Altyazı ile film izlemek = to watch films with subtitles
• Altyazılı film izlemek = to watch subtitled films

The difference is small:

• altyazı ile emphasizes using subtitles
• altyazılı describes the films themselves as subtitled

In many contexts, native speakers would probably prefer Altyazılı film izlemek.

It means the language being learned, and the exact language is understood from context.

So if the learner is studying Turkish, then dili geliştirmek would be understood as improving Turkish. In another context, it could mean English, German, or any other language.

If you want to be more specific, Turkish can say:

• Türkçeyi geliştirmek = to improve Turkish
• İngilizceyi geliştirmek = to improve English

Yes, it is natural and understandable.

A few slightly different versions that also sound natural are:

• Altyazılı film izlemek dili geliştirmek için faydalı sayılır.
• Altyazılı film izlemek, dili geliştirmek için faydalıdır.
• Altyazılı film izlemek dil becerilerini geliştirmek için faydalıdır.

The original sentence is perfectly fine. The main thing a learner should notice is how Turkish builds the idea through:

• an infinitive subject: izlemek
• a purpose phrase: geliştirmek için
• a general evaluative predicate: faydalı sayılır

Sometimes, yes, depending on style.

Possible alternatives include:

• altyazıyla = same meaning, just ile attached
• altyazılı = subtitled
• altyazı kullanarak = by using subtitles

For example:

• Altyazıyla film izlemek...
• Altyazılı film izlemek...
• Altyazı kullanarak film izlemek...

All of these are possible, but they are not identical in nuance:

• ile / -yla = with
• altyazılı = subtitled
• kullanarak = by using

So the original sentence is one of several natural ways to express the idea.