Je passe par la pharmacie avant de rentrer à la maison.

Passer par is a very common expression meaning to go by, to go via, or to stop by on the way.

So:

• Je vais à la pharmacie = I’m going to the pharmacy.
• Je passe par la pharmacie = I’m stopping by the pharmacy / going via the pharmacy before doing something else.

In this sentence, par suggests the pharmacy is one step in the route before going home.

Here, passer par means something like to go by or to stop off at a place during a trip.

It does not necessarily mean you are only physically passing in front of it. In everyday French, it often implies a short visit or stop.

Examples:

• Je passe par la boulangerie. = I’m stopping by the bakery.
• On passe par Paris. = We’re going via Paris.

So in this sentence, the speaker is saying they will go to the pharmacy before heading home.

In French, la pharmacie usually refers to the pharmacy as a place or shop. That is the most natural choice here.

• la pharmacie = the pharmacy
• le pharmacien / la pharmacienne = the pharmacist

You can say chez le pharmacien, but that focuses more on the professional person, like at the pharmacist’s place. In most everyday situations, French speakers simply say à la pharmacie or par la pharmacie.

Because avant must be followed by de when the next verb is in the infinitive.

So the pattern is:

• avant de + infinitive

Examples:

• avant de partir = before leaving
• avant de manger = before eating
• avant de rentrer = before going home / before returning

So avant rentrer is not correct.

Use avant de + infinitive when the subject of both actions is the same.

In this sentence, the same person does both actions:

• Je passe...
• je rentre...

So French uses avant de rentrer.

If the subject changes, French uses avant que + subjunctive:

• Je passe à la pharmacie avant que tu rentres.
  = I’m stopping by the pharmacy before you go home.

So:

• same subject → avant de + infinitive
• different subject → avant que + subjunctive

Rentrer often means to go back, to come back, or very often to go home.

In this sentence, rentrer à la maison means to go home or to return home.

Compared with retourner:

• rentrer often suggests returning to where you belong, especially home
• retourner is more generally to return/go back

In everyday French, rentrer à la maison is very natural for go home.

Because maison is a feminine noun, so à + la maison becomes à la maison.

This expression means home or to the house/home.

Compare:

• à la maison = at home / home
• chez moi = at my place / to my place

Both can sometimes translate as home, but:

• rentrer à la maison = go home
• rentrer chez moi = go back to my place

Both are common, but à la maison is the exact form used in your sentence.

Passe is the present indicative form of passer for je.

• je passe
• tu passes
• il/elle passe

Even though it is grammatically present tense, French often uses the present to talk about:

• a current action
• a routine
• a near future action

So depending on context, Je passe par la pharmacie avant de rentrer à la maison could mean:

• I’m stopping by the pharmacy before going home.
• I stop by the pharmacy before going home.
• I’ll stop by the pharmacy before going home.

French present tense is often broader than English simple present.

In French, after de, the infinitive comes directly:

• de rentrer
• de manger
• de partir

English often uses to + verb, but French infinitives do not need a separate word equivalent to English to in this structure.

So:

• avant de rentrer = before going home / before returning home

You should think of de rentrer as a normal French grammar pattern, not as a word-for-word match with English.

Yes, in some contexts, passer à la pharmacie can sound natural, especially in informal speech, meaning to stop at the pharmacy.

But passer par la pharmacie emphasizes the idea of the pharmacy being a stop on the way somewhere else.

So the nuance is roughly:

• passer à la pharmacie = stop at the pharmacy
• passer par la pharmacie = go by / stop by the pharmacy on the way

In your sentence, par works especially well because the speaker then says avant de rentrer à la maison.

Yes, it is natural and idiomatic.

French speakers often use passer par for places they are going by or stopping at during a trip:

• Je passe par la banque.
• Je passe par le supermarché.
• On passe par chez toi ?

So this sentence sounds like normal everyday French.

Yes, you could say:

Je passe par la pharmacie avant de rentrer chez moi.

That is also very natural.

The difference is small:

• à la maison = home / the house
• chez moi = my place / my home

Chez moi sounds a bit more personal, while à la maison is a very standard general way to say home.

A good way to learn this sentence is to remember it in useful pieces:

• passer par + place = to stop by / go via
• avant de + infinitive = before doing something
• rentrer à la maison = to go home

So you can reuse them in many sentences:

• Je passe par le supermarché avant de rentrer à la maison.
• Elle passe par la banque avant de partir.
• On mange avant de sortir.

Learning these chunks is often more useful than translating word by word.