hyudaeponeul bakkugi jeone yeonrakcheoreul baegeophae duseyo.

바꾸기 전에 means before changing (it) or before you change (your phone).

Here, -기 turns the verb 바꾸다 (to change, replace) into a noun-like form:

• 바꾸다 = to change
• 바꾸기 = changing / the act of changing
• 바꾸기 전에 = before changing

Korean often uses verb + -기 전에 to mean before doing something.

을 is the object marker. It shows that 휴대폰 is the thing being changed.

So:

• 휴대폰을 바꾸다 = to change/replace a cellphone

Because 휴대폰 ends in a consonant sound, it takes 을 rather than 를.

They are very similar in everyday meaning.

• 휴대폰 = cellphone / mobile phone
• 핸드폰 = cellphone, also very common in speech

A small nuance:

• 휴대폰 is often considered a bit more standard
• 핸드폰 is very common in casual conversation

In this sentence, 휴대폰 sounds perfectly natural.

연락처 means contact information or contacts.

In the context of a phone, it usually means your saved contacts, such as:

• phone numbers
• names
• possibly other stored contact details

So 연락처를 백업하다 means to back up your contacts.

This is an important nuance.

• 백업하세요 = Please back them up.
• 백업해 두세요 = Please back them up in advance / please make sure they are backed up beforehand.

The pattern -아/어 두다 often means:

• doing something ahead of time
• doing something and leaving it in that completed state for later benefit

So 백업해 두세요 fits very well with before changing your phone. It suggests preparation.

Because 두다 is acting as an auxiliary verb here.

• 백업해 두세요 = back it up and keep it done / have it backed up

In standard spacing, Korean often writes the main verb and auxiliary separately:

• 해 두세요
• 써 놓다
• 읽어 보다

In casual writing, learners may sometimes see different spacing, but 백업해 두세요 is the standard form.

-세요 makes the sentence a polite request, suggestion, or instruction.

So 백업해 두세요 means something like:

• Please back up your contacts in advance
• Make sure to back up your contacts

It is polite and commonly used in advice, instructions, customer service, and notices.

Yes. The subject is understood from context.

Korean often omits subjects when they are obvious. In this kind of sentence, the implied meaning is:

• Before you change your phone, please back up your contacts.

There is no need to say 당신은 or another word for you.

Yes. In English, both are possible depending on context.

• 휴대폰을 바꾸다 often means to replace your phone or switch to a new phone
• Literally, it is to change your cellphone

In this sentence, the natural meaning is probably before you switch phones / before replacing your phone.

전에 is the normal word used after a noun or noun-like phrase to mean before.

Examples:

• 식사 전에 = before the meal
• 자기 전에 = before sleeping
• 휴대폰을 바꾸기 전에 = before changing your phone

It is one of the most common and useful ways to express before doing something.

Yes, but there is a small difference in usage.

• 바꾸기 전에 is the full adverbial form: before changing
• 바꾸기 전 is often used directly before a noun: the time before changing, prior to changing

For example:

• 휴대폰을 바꾸기 전에 연락처를 백업해 두세요. = natural sentence
• 휴대폰을 바꾸기 전 확인사항 = things to check before changing your phone

So in this sentence, 전에 is the more natural choice.

Yes. 백업하다 is a very common loanword-based verb in Korean.

Korean often takes an English-based noun and combines it with 하다:

• 백업하다 = to back up
• 체크하다 = to check
• 다운로드하다 = to download

So 연락처를 백업해 두세요 is very natural modern Korean.

It sounds polite and neutral.

It would fit well in places like:

• tech support instructions
• store advice
• a notice on a website
• someone politely advising another person

It is not extremely formal, but it is definitely polite. A casual version might be:

• 휴대폰 바꾸기 전에 연락처 백업해 둬.

A more formal written style might be adjusted slightly, but the given sentence is very natural for everyday polite Korean.