Θα περάσω από το φαρμακείο πριν γυρίσω σπίτι, γιατί χρειάζομαι σιρόπι.

Yes, περνάω / περνώ often means to pass, but in the expression περνάω από + place, it very often means to stop by or to drop in at a place.

So:

• Θα περάσω από το φαρμακείο = I’ll stop by the pharmacy
• not just I will pass the pharmacy

This is a very common everyday use of περνάω.

Because Greek uses aspect very strongly in the future.

• θα περάσω = a single, complete action
• θα περνάω = a repeated or ongoing action

In this sentence, the speaker means one specific stop at the pharmacy, so Greek uses the perfective form:

• θα περάσω

If you said:

• Θα περνάω από το φαρμακείο κάθε Δευτέρα

that would mean I’ll be stopping by the pharmacy every Monday.

So περάσω here is the natural choice for a one-time future event.

Yes, από often means from, but with περνάω, the combination περνάω από is idiomatic and means:

• go by
• stop by
• drop in at

So:

• περνάω από το φαρμακείο = I stop by the pharmacy

You should learn περνάω από as a set expression.

Compare:

• Πέρασα από τον φίλο μου. = I stopped by my friend’s place.
• Πέρασα από την τράπεζα. = I stopped by the bank.

Greek uses the definite article much more often than English.

So where English says:

• I’m going to the pharmacy
• or even sometimes just I’m going to pharmacy in a looser explanation

Greek normally says:

• το φαρμακείο

After a preposition, this is very normal:

• από το φαρμακείο
• στο φαρμακείο
• για το φαρμακείο

In Greek, leaving the article out here would sound unnatural.

Because after πριν meaning before, Greek does not use θα.

Instead, Greek uses a subjunctive-type form:

• πριν γυρίσω = before I return / before I go back

So:

• Θα περάσω από το φαρμακείο πριν γυρίσω σπίτι

is the normal structure.

A good rule is:

• θα in the main clause
• no θα after πριν

Γυρίσω is the perfective form of γυρίζω, used here because the speaker means one completed return home.

So:

• πριν γυρίσω σπίτι = before I go back home

If you used the imperfective idea instead, it would suggest something more ongoing or repeated, which does not fit as well here.

As for επιστρέψω:

• πριν επιστρέψω σπίτι is also possible
• it sounds a bit more formal or more literal as return

γυρίζω / γυρίσω is very common in everyday speech.

Because σπίτι often works like home in English.

Greek commonly says:

• πάω σπίτι = I’m going home
• γυρίζω σπίτι = I’m returning home
• μένω σπίτι = I stay home

So in this sentence:

• πριν γυρίσω σπίτι = before I go back home

If you say στο σπίτι, that is also possible, but it feels more like to the house / at the house / at home depending on context.

So the bare σπίτι here is completely natural and idiomatic.

Yes. Γιατί can mean both:

• because
• why

The meaning depends on the sentence.

Here it clearly means because:

• γιατί χρειάζομαι σιρόπι = because I need syrup

As a question word, it can mean why:

• Γιατί φεύγεις; = Why are you leaving?

So the word is the same, but the grammar and punctuation show the meaning.

Because Greek often leaves out the indefinite article where English might say a or some.

So:

• χρειάζομαι σιρόπι can mean I need syrup or I need some syrup
• in this pharmacy context, it often means I need some medicinal syrup / cough syrup

If you say:

• χρειάζομαι ένα σιρόπι

that sounds more like I need a syrup, possibly one item, one bottle, or one specific product.

Without ένα, the sentence sounds more general and natural here.

Not by itself. Σιρόπι literally means syrup.

But in the context of a pharmacy:

• χρειάζομαι σιρόπι

will often be understood as I need medicinal syrup, very often cough syrup.

If you want to be more explicit, you could say something like:

• σιρόπι για τον βήχα = cough syrup

So the exact meaning comes from the situation.

Good observation. The nouns here are mostly neuter, and in neuter nouns the nominative and accusative are often the same in form.

In this sentence:

• το φαρμακείο is accusative after από
• σπίτι is used in an adverbial/directional way
• σιρόπι is accusative as the object of χρειάζομαι

They look unchanged because neuter forms are often identical in these cases.

So the grammar is there, but the endings are not very visible in this sentence.

Yes, but the nuance changes.

• Θα πάω στο φαρμακείο = I will go to the pharmacy
• Θα περάσω από το φαρμακείο = I will stop by the pharmacy

The second one often suggests:

• it is just one stop on the way
• the visit may be brief
• the pharmacy is not necessarily the main destination

So in this sentence, περάσω από fits nicely with before I go home.