Başvuru numarasını kaybetmeyin.

The dictionary form is kaybetmek, which means to lose.

In this sentence, kaybetmeyin means don’t lose.

It breaks down like this:

• kaybet- = lose
• -me- = negative marker
• -yin = imperative ending used for you plural or polite you singular

So kaybetmeyin literally means do not lose.

Because başvuru numarasını is the direct object of the verb, and it is definite/specific.

In Turkish, a specific direct object usually takes the accusative ending.

Here is the structure:

• başvuru numarası = application number
• başvuru numarasını = the application number

The final -nı is the accusative ending here.

So:

• başvuru numarası = application number
• başvuru numarasını kaybetmeyin = don’t lose the application number

This is because başvuru numarası is a very common Turkish noun compound structure.

It is made like this:

• başvuru = application
• numara = number
• numarası = its number / number of the application

In Turkish, compounds like this often use a suffix on the second noun:

• öğrenci kartı = student card
• telefon numarası = phone number
• başvuru numarası = application number

That -sı does not necessarily mean a real possessor like his/her in English. In compounds, it is just part of the standard structure.

Then, when you add the accusative, it becomes:

• numarası
  • -nı → numarasını

The n is a buffer consonant.

The n is a buffer consonant.

When a Turkish word already ends in a possessive-type suffix such as -sı, and another vowel-initial ending is added after it, Turkish often inserts n to make pronunciation smoother.

So:

• numarası
• numarası + ı
• becomes numarasını

This is very normal in Turkish grammar.

It can be either:

• more than one person: you all
• one person politely/formally: you

So kaybetmeyin can mean:

• Don’t lose it, everyone
• Don’t lose it, sir/ma’am

If you wanted the informal singular form, you would usually say:

• kaybetme = don’t lose

So the sentence could become:

• Başvuru numarasını kaybetme. = Don’t lose the application number.
  (said to one person informally)

Because Turkish normally uses Subject–Object–Verb word order.

So the basic order is often:

• object + verb

Here:

• Başvuru numarasını = the application number
• kaybetmeyin = don’t lose

So the verb naturally comes at the end.

This is one of the biggest differences from English, where we usually say:

• Don’t lose the application number

In Turkish, the equivalent order is more like:

• The application number don’t lose

Başvuru means application in the sense of a formal request, registration, or submission.

For example:

• üniversite başvurusu = university application
• iş başvurusu = job application
• başvuru formu = application form

So başvuru numarası means the number connected to an application, such as a registration number, reference number, or application number.

A rough pronunciation guide is:

bash-voo-roo noo-mah-rah-suh-nuh kahy-bet-mey-in

A few useful sounds:

• ş sounds like sh
• ı is a vowel that does not exist in English; it is a dull, neutral vowel, something like an unstressed uh but farther back in the mouth
• j is not present here, but remember that Turkish spelling is usually very regular

A more syllable-by-syllable breakdown:

• Baş-vu-ru
• nu-ma-ra-sı-nı
• kay-bet-me-yin

You can understand it word for word, but it is better to learn it as a set phrase.

Word for word, it is something like:

• application number / number of the application

But in natural English, the best translation is usually:

• application number
• sometimes reference number, depending on context

This is very common with Turkish noun compounds: the literal structure is a little different from English, but the meaning is straightforward once you recognize the pattern.

Yes. Kaybetmeyin is already polite or plural, but it is still a direct instruction.

A softer version could be:

• Lütfen başvuru numarasını kaybetmeyin. = Please don’t lose the application number.

Adding lütfen makes it more polite.

Depending on context, Turkish can also use more indirect wording, but for signs, instructions, and official communication, Başvuru numarasını kaybetmeyin is perfectly natural.