Başvuru formunu doldururken kimlik bilgilerini dikkatle yazmalısın.

It breaks down like this:

• başvuru = application
• formu = its form / form of application → together, başvuru formu means application form
• -nu = accusative ending added because it is the specific direct object of the sentence

So:

• başvuru formu = application form
• başvuru formunu = the application form (as the object)

A useful point: in Turkish, noun combinations like başvuru formu, kimlik kartı, telefon numarası often use this structure, where the second noun takes a suffix.

Because both are definite direct objects.

In Turkish, when the object is specific, it usually takes the accusative ending.

Here:

• başvuru formunu = the application form
• kimlik bilgilerini = the identity details / identification information

Both refer to specific things, not just any form or any information.

Compare:

• form doldurmak = to fill out a form, in general
• formu doldurmak = to fill out the form, a specific one

And similarly:

• bilgi yazmak would sound very general or unnatural here
• bilgilerini yazmak = write your/the relevant details

Because formu is already part of the noun phrase başvuru formu.

Here is the structure:

• başvuru formu = application form
• then you add the accusative ending
• after a possessed form like formu, Turkish uses a buffer n
• so: formu + nu → formunu

This same pattern appears in many words:

• kitabı
  • nı → kitabını
• arabasını
• numarasını

So formunu is not one single mysterious ending; it is:

• formu
  • -nu

doldururken means while filling out or when filling out.

It comes from the verb:

• doldurmak = to fill out / fill in

And the ending -urken creates a while doing... meaning.

So:

• doldururken = while filling out
• okurken = while reading
• konuşurken = while speaking
• yürürken = while walking

In this sentence, it connects two actions happening at the same time:

• filling out the application form
• writing the identity details carefully

For many learners, the easiest way is to treat -urken as a pattern meaning while doing.

But if you want the technical breakdown, doldururken historically contains an aorist-like element before -ken.

So the form is related to:

• doldurur
  • -ken → doldururken

You do not need to translate the -ur- separately here. The whole form simply means:

• while filling out

A practical tip: learn these as chunks:

• gelirken = while coming
• giderken = while going
• yaparken = while doing
• doldururken = while filling out

It means identity details, identification information, or personal ID information.

Breakdown:

• kimlik = identity / ID
• bilgileri = information/details
• bilgilerini = that same phrase in the accusative

More precisely:

• bilgi = information / piece of information
• bilgiler = pieces of information / details
• bilgileri = the information/details of identity → identity details
• bilgilerini = identity details as a specific object

This is a very natural Turkish expression. Even though English often uses singular information, Turkish often uses plural bilgiler when talking about someone’s details.

Because dikkatle is functioning as an adverb here: carefully.

• dikkat = attention, care
• dikkatle = with attention / carefully

So:

• dikkatle yazmak = to write carefully

By contrast:

• dikkatli usually means careful as an adjective

Examples:

• Dikkatli bir öğrenci. = A careful student.
• Dikkatle oku. = Read carefully.

In everyday Turkish, both dikkatle and dikkatli bir şekilde can express a similar idea, but dikkatle is the more compact and natural choice here.

yazmalısın is built from:

• yaz- = write
• -malı / -meli = should, must, need to
• -sın = you (singular)

So:

• yazmalısın = you should write / you must write

The -malı/-meli form can express:

• advice
• obligation
• instruction
• mild necessity

In this sentence, it sounds like an instruction or strong recommendation.

Related forms:

• yazmalıyım = I should write
• yazmalı = he/she should write
• yazmalıyız = we should write
• yazmamalısın = you should not write

It is contained in the verb ending -sın in yazmalısın.

Turkish usually drops subject pronouns when the verb already makes the subject clear.

So:

• yazmalısın already means you should write
• adding sen is possible, but only for emphasis

For example:

• Kimlik bilgilerini dikkatle yazmalısın. = You should write your identity details carefully.
• Sen kimlik bilgilerini dikkatle yazmalısın. = You should write your identity details carefully.
  This sounds more emphatic.

Usually, yes.

In this sentence, the natural interpretation is that the same person:

• fills out the application form
• writes the identity details carefully

That person is you, shown by yazmalısın.

So the sentence means something like:

• while you are filling out the application form, you should write the identity details carefully

Turkish often leaves the subject unstated when it is understood from context and verb endings.

This word order is very natural, but Turkish word order is somewhat flexible.

The sentence is:

• Başvuru formunu doldururken kimlik bilgilerini dikkatle yazmalısın.

Why it sounds natural:

• Başvuru formunu doldururken sets the time/context first
• kimlik bilgilerini gives the object
• dikkatle gives the manner
• yazmalısın comes at the end, where Turkish verbs often go

You can move parts around for emphasis, but the original is a very standard order.

For example:

• Kimlik bilgilerini başvuru formunu doldururken dikkatle yazmalısın.

This is still understandable, but the original sounds smoother as a neutral sentence.

Because the natural collocations are different:

• you fill out a form → form doldurmak
• you write details/information → bilgi yazmak, adını yazmak, adresini yazmak

So in this sentence:

• başvuru formunu doldurmak = fill out the application form
• kimlik bilgilerini yazmak = write the identity details

This is very similar to English, where we also often say:

• fill out the form
• write your name/details

So Turkish is using two different verbs because two different actions are being described.