Apoteka je blizu škole, ali ja sada idem u prodavnicu.

Because blizu normally requires the genitive case.

• Basic form: škola = school
• Genitive singular: škole = of the school / near the school

So:

• blizu škole = near the school

This is one of those patterns you simply learn together:

• blizu + genitive
• pored + genitive
• kod + genitive

So a learner should remember blizu škole, not blizu škola.

Because Serbian uses different cases after u depending on whether you are talking about:

• movement toward/into a place → accusative
• location in a place → locative

Here the verb is idem = I am going, so there is movement. That is why Serbian uses:

• u prodavnicu = to the store / into the store

Compare:

• Idem u prodavnicu. = I’m going to the store.
• U prodavnici sam. = I’m in the store.

So u can mean both in and to, and the case tells you which meaning is intended.

Je is the 3rd person singular present tense of biti = to be.

So:

• Apoteka je blizu škole. = The pharmacy is near the school.

In English, you also need is here, and Serbian does too.

A very short present-tense set of forms of biti is:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is
• mi smo = we are
• vi ste = you are
• oni/one/ona su = they are

Yes. Idem already tells you the subject is I, so ja is not grammatically necessary.

Serbian often omits subject pronouns when the verb form already makes the subject clear. But speakers may include ja for:

• emphasis
• contrast
• clarity

In this sentence, ja sounds natural because of the contrast created by ali = but:

• Apoteka je blizu škole, ali ja sada idem u prodavnicu.

This gives a feeling like:

• …but I am going to the store now

So ja adds contrast, not basic meaning.

Ali means but.

It connects two ideas in contrast:

• The pharmacy is near the school
• but I am going to the store now

So the speaker is not saying the pharmacy is far away. Instead, the second clause contrasts with the first idea: even though the pharmacy is near the school, the speaker is going somewhere else.

Yes, Serbian word order is fairly flexible, and sada can often move around.

In this sentence:

• ja sada idem u prodavnicu

the position of sada is natural and neutral: I am going to the store now.

You may also hear:

• Ja idem sada u prodavnicu.
• Sada idem u prodavnicu.

These are all possible, but the emphasis can shift slightly depending on placement.

Very roughly:

• ja sada idem... = neutral, with now in a natural place
• sada idem... = stronger focus on now
• ja idem u prodavnicu sada = possible, but often sounds more marked or conversational depending on context

So the sentence as given is a standard, natural version.

Yes. Idem is the 1st person singular present tense of ići = to go.

So:

• idem = I go / I am going

In Serbian, the present tense often covers both:

• habitual meaning: I go
• current action: I am going

In this sentence, because of sada = now, the meaning is clearly:

• I am going now

A few present-tense forms of ići are:

• idem = I go
• ideš = you go
• ide = he/she/it goes
• idemo = we go
• idete = you go
• idu = they go

For learners, it is easiest to think of blizu here as a word that behaves like a preposition-like expression and takes the genitive:

• blizu škole = near the school

In other contexts, blizu can also behave adverbially:

• Živim blizu. = I live nearby.

So depending on the sentence, it can function a bit differently. But in blizu škole, the important practical rule is:

• blizu + genitive

Yes. Apoteka is the subject of the first clause, so it is in the nominative singular.

• apoteka = pharmacy
• nominative singular feminine noun ending in -a

The clause structure is:

• Apoteka = subject
• je = is
• blizu škole = near the school

So apoteka stays in its basic dictionary form here.

Because prodavnica is a feminine noun ending in -a, and for many feminine nouns of that type, the accusative singular ends in -u.

So:

• nominative: prodavnica
• accusative: prodavnicu

That is why we get:

• idem u prodavnicu

The same pattern appears with many other feminine nouns:

• škola → školu
• kuća → kuću
• banka → banku

This is a very common pattern in Serbian.

Yes, absolutely:

• Apoteka je blizu škole, ali sada idem u prodavnicu.

That is also correct and natural.

The version with ja just adds a little extra contrast or emphasis. Serbian often leaves pronouns out unless there is a reason to highlight them.

So both are good:

• ... ali ja sada idem u prodavnicu.
• ... ali sada idem u prodavnicu.

It can suggest either, depending on context.

With a verb of motion and the accusative, u + accusative basically means movement into a place, but in natural English this is often translated as to:

• Idem u prodavnicu. = I’m going to the store.

English usually does not insist on the difference unless it matters. Serbian marks the direction more clearly through the case.

So literally there is an idea of entering the store, but the normal English translation is simply to the store.

Yes. That is very normal in Serbian.

The difference is shown by the case, not by changing the preposition:

• u + locative = in
• u + accusative = to / into

Examples:

• u školi = in the school

• u školu = to the school

• u prodavnici = in the store
• u prodavnicu = to the store

This is one of the most important patterns for place expressions in Serbian.

It is grammatically natural Serbian.

A native speaker could absolutely say it. That said, it also works very well as a teaching sentence because it includes several useful features:

• je = present of to be
• blizu + genitive
• ali = but
• sada = now
• idem = present tense of ići
• u + accusative for motion

So it is both natural and pedagogically useful.