Misafirler gelmeden önce teyzem mutfakta börek açıyor.

It is a very common Turkish time expression meaning before coming / before someone comes.

Breakdown:

• gel- = come
• -meden = a non-finite form that, in this pattern, helps express before doing
• önce = before

So misafirler gelmeden önce means before the guests come / arrive.

A useful thing to remember is that -meden önce works as a set pattern:

• yemeden önce = before eating
• gitmeden önce = before going
• uyumadan önce = before sleeping

Because önce is usually a postposition in Turkish. That means it comes after the word or clause it relates to.

So Turkish says:

• dersten önce = before class
• yemekten önce = before the meal
• misafirler gelmeden önce = before the guests come

English uses before in front of the phrase, but Turkish usually places önce after it.

The subject is misafirler.

So in misafirler gelmeden önce, the ones who are coming are the guests.

In Turkish, this kind of subordinate verb form does not usually show person the way a normal finite verb does, so the subject is understood from the noun placed with it.

Because misafirler is the subject of the subordinate clause gelmeden önce.

It is the guests who are arriving, so the plain form misafirler is correct.

If it were a direct object, you might expect different marking depending on context, but here it is simply the doer of gelmek.

Teyzem means my aunt, specifically my mother’s sister.

Breakdown:

• teyze = maternal aunt
• -m = my

So:

• teyze = aunt
• teyzem = my aunt

This is worth noticing because Turkish often distinguishes family relations more specifically than English does. For example:

• teyze = mother’s sister
• hala = father’s sister

Because mutfakta means in the kitchen.

Breakdown:

• mutfak = kitchen
• -ta = in / at

This is the locative suffix. Its forms are -da / -de / -ta / -te, depending on vowel harmony and consonant voicing.

Here it becomes -ta:

• mutfak → mutfakta

So mutfakta = in the kitchen.

Yes, açmak often means to open, but in cooking it can also mean to roll out or stretch out dough.

So börek açmak does not mean to open a börek. It refers to preparing the pastry/dough for börek by rolling it out thin.

This is a good example of how a very common Turkish verb can have a more specialized meaning in certain contexts.

Because the main subject of the sentence is teyzem, not misafirler.

The sentence has two parts:

• Misafirler gelmeden önce = before the guests arrive
• teyzem mutfakta börek açıyor = my aunt is rolling out börek dough in the kitchen

So the verb açıyor agrees with teyzem, which is singular.

Açıyor is the present continuous form.

Breakdown:

• aç- = open / roll out
• -ıyor = present continuous

So it can mean:

• is rolling out
• is making
• sometimes, depending on context, a repeated or currently relevant action

In this sentence, the most natural reading is that your aunt is in the middle of doing this before the guests arrive.

Because Turkish usually does not use articles the way English does.

So:

• misafirler can mean guests or the guests
• mutfakta can mean in a kitchen or in the kitchen

The exact meaning comes from context.

That is why the English translation may include the, even though Turkish has no separate word for it here.

Yes. Turkish word order is fairly flexible.

The given sentence is natural:

• Misafirler gelmeden önce teyzem mutfakta börek açıyor.

But you could also say:

• Teyzem misafirler gelmeden önce mutfakta börek açıyor.

Both are grammatical. The difference is mainly focus and style.

Putting Misafirler gelmeden önce first makes the time setting come first, which is very common in Turkish.

Not necessarily.

Turkish often does not require a comma after a short time clause like this, so the version without a comma is completely natural:

• Misafirler gelmeden önce teyzem mutfakta börek açıyor.

You may sometimes see a comma in writing for clarity or rhythm:

• Misafirler gelmeden önce, teyzem mutfakta börek açıyor.

But it is not required here.