Между прочим, я заодно купил хлеб, когда ходил в аптеку.

Here между прочим is a set expression meaning by the way or incidentally. It often introduces an extra piece of information that is not the main point.

So in this sentence, the speaker is casually adding: By the way, I also bought bread...

Even though the words literally look like between other things, you should usually learn между прочим as a fixed phrase.

Заодно means something like:

• while I was at it
• at the same time
• since I was already there / already doing that

It suggests that the speaker combined errands or actions for convenience.

So:

• Я заодно купил хлеб = I bought bread too, while I was at it
• Я тоже купил хлеб = I also bought bread
• Я также купил хлеб = I also bought bread / I bought bread as well

The difference is that заодно adds the idea of doing it in connection with another action.

Купил is perfective, so it presents the purchase as a completed result: the bread was bought.

That fits this sentence well, because the speaker is talking about one finished action during a particular outing.

• купил = bought / ended up buying
• покупал = was buying, used to buy, or sometimes bought with focus on the process/repetition rather than the result

Here, the result matters: the speaker successfully got the bread.

This is a very common Russian motion-verb question.

Ходил в аптеку usually suggests a there-and-back trip: the speaker went to the pharmacy and came back. In everyday Russian, this is often how people talk about having gone somewhere on an errand.

By contrast:

• шёл в аптеку = was walking/going to the pharmacy
  Focuses on the movement in progress.
• пошёл в аптеку = set off/went to the pharmacy
  Focuses on the start of the trip.
• ходил в аптеку = went to the pharmacy (and back)
  Focuses on the completed outing as an event.

In this sentence, the speaker is referring to that whole errand, so ходил is very natural.

Because в аптеку uses the accusative case to show motion toward a destination.

• в аптеку = to the pharmacy
• в аптеке = in/at the pharmacy

So:

• ходил в аптеку = went to the pharmacy
• был в аптеке = was in the pharmacy

This is a standard Russian pattern:

• куда? (where to?) → usually accusative
• где? (where?) → usually prepositional

Not necessarily. In fact, the most natural reading is that the speaker bought bread during the same outing, not necessarily inside the pharmacy.

Because of заодно, the sentence suggests something like:

• I went to the pharmacy, and while I was out, I picked up bread too.

So the bread could have been bought at a nearby store, supermarket, kiosk, etc.

In this sentence, когда can be understood as when or while, depending on how natural English sounds in context.

• когда ходил в аптеку = when I went to the pharmacy
• or more loosely: while I was going to / while I was out on a trip to the pharmacy

The main point is that the bread-buying happened in connection with that trip.

Russian когда is very broad and often covers both the simple English idea of when and the looser time-background idea that English might sometimes express with while.

In Russian, past tense singular verbs agree with the speaker's or subject's gender.

• купил, ходил = masculine
• купила, ходила = feminine
• купило, ходило = neuter
• купили, ходили = plural

So this sentence suggests that the speaker is male.

If the speaker were female, it would be:

Между прочим, я заодно купила хлеб, когда ходила в аптеку.

Yes. Russian word order is fairly flexible, and changing it usually affects emphasis rather than the basic meaning.

For example:

• Между прочим, я заодно купил хлеб, когда ходил в аптеку.
• Между прочим, когда я ходил в аптеку, я заодно купил хлеб.
• Я, между прочим, заодно купил хлеб, когда ходил в аптеку.

These all mean roughly the same thing, but the focus shifts slightly.

The original sentence sounds natural and conversational. Putting заодно near купил works well because it directly modifies that action.

It could often be omitted if the subject is already clear from context, because Russian frequently drops subject pronouns.

So you could say:

Между прочим, заодно купил хлеб, когда ходил в аптеку.

That would sound natural if it is already obvious who is speaking.

However, keeping я is also perfectly normal. It can make the sentence a little clearer or slightly more explicit.

Usually, yes: аптека is the normal Russian word for pharmacy, chemist's, or drugstore.

Depending on the variety of English:

• American English: pharmacy / drugstore
• British English: chemist's

So ходил в аптеку is a very ordinary phrase meaning went to the pharmacy/chemist's.

Russian has no articles like English a/an/the. So nouns like хлеб and аптеку appear without any separate word for a or the.

Whether English should use a or the depends on context:

• купил хлеб = bought bread / bought some bread
• в аптеку = to the pharmacy / to a pharmacy

In this sentence, English usually uses the pharmacy because it sounds like a specific errand the speaker had in mind.