yakuzaisi ha kono kusuri no nomikata wo setumeisite kuremasita.

は marks 薬剤師 as the topic: As for the pharmacist...

In this sentence, the pharmacist is also effectively the subject of the action, but は presents them as the topic of the statement rather than emphasizing who did it.

If you used が instead, it would put more focus on the pharmacist as the one who explained it, as if answering Who explained it?

So:

• 薬剤師は... = speaking about the pharmacist
• 薬剤師が... = emphasizing that the pharmacist is the one who did it

Both can be grammatical, but は is very natural here.

この modifies 薬.

So:

• この薬 = this medicine
• この薬の飲み方 = how to take this medicine

It does not directly modify 飲み方. The structure is:

• この → 薬
• この薬 → の飲み方

Here, の connects この薬 and 飲み方.

A very literal breakdown would be:

• この薬 = this medicine
• 飲み方 = way of taking/drinking
• この薬の飲み方 = this medicine's way of taking

But in natural English, that becomes how to take this medicine.

So this の is not just simple possession. It often shows a relationship like:

• of
• for
• related to
• concerning

In this sentence, it means something like the method for taking this medicine.

方 in this pattern means way of doing something.

It attaches to the verb stem:

• 飲む → 飲み
  • 方 = 飲み方 = way of drinking/taking
• 食べる → 食べ方 = way of eating
• 使う → 使い方 = way of using

So 飲み方 means how to take it or the proper way to take it.

This is a very common pattern in Japanese.

In Japanese, 飲む is the normal verb used for taking medicine, even if it is a pill or tablet.

So Japanese commonly says:

• 薬を飲む = take medicine

This may feel strange to English speakers, because English usually says take medicine rather than drink medicine for pills. But in Japanese, 飲む is the standard word here.

So 飲み方 is exactly what you would expect in a sentence about medicine instructions.

Because the thing being explained is the whole phrase この薬の飲み方.

The pharmacist did not simply explain the medicine. The pharmacist explained how to take the medicine.

So:

• この薬 is part of the modifier
• この薬の飲み方 is the full object
• therefore を comes after 飲み方

That is why the sentence is:

• この薬の飲み方を説明してくれました

not something like:

• この薬を飲み方説明して... which would be incorrect

This is 説明する plus くれる.

Breakdown:

• 説明する = to explain
• 説明して = te-form of 説明する
• くれました = gave me/us the favor of doing it

So 説明してくれました means more than just explained. It adds the nuance that the action was done for the speaker or for someone in the speaker's group, and that it was beneficial from the speaker's point of view.

So compared with plain 説明しました:

• 説明しました = explained
• 説明してくれました = explained it for me/us, kindly explained it to me/us

With 〜てくれる, the receiver is usually the speaker or someone close to the speaker, such as the speaker's family or group.

Japanese often leaves that person unstated when it is obvious from context.

So this sentence naturally implies something like:

• The pharmacist explained how to take this medicine to me
• The pharmacist explained how to take this medicine for us

If you wanted to say it explicitly, you could say:

• 薬剤師は私にこの薬の飲み方を説明してくれました。

But very often 私に is omitted because it is already understood.

Because 〜てくれる is a grammar pattern.

Pattern:

• verb in te-form + くれる

It means that someone does that action for the speaker's benefit.

So:

• 説明する → 説明して
• 説明してくれる = to explain for me/us

This is not two separate actions. It is one combined expression.

The te-form links the main action to the helping/benefactive verb くれる.

Yes. 薬剤師はこの薬の飲み方を説明しました is grammatical and natural.

The difference is nuance:

• 説明しました = a neutral statement of fact
• 説明してくれました = the explanation was given to me/us, with a helpful or appreciated nuance

In a real-life situation, especially from the patient's point of view, 説明してくれました often sounds more natural because it reflects that the pharmacist did something helpful for the speaker.

Because the sentence is describing something that already happened.

The pharmacist already gave the explanation, so the sentence uses the past polite form:

• くれます = gives / will give
• くれました = gave

Since くれる is the final verb, it carries the tense and politeness for the whole predicate:

• 説明してくれました = explained for me/us

Yes, it is polite because it ends with ました.

Polite version:

• 薬剤師はこの薬の飲み方を説明してくれました。

Casual version:

• 薬剤師はこの薬の飲み方を説明してくれた。

The polite version is appropriate in normal conversation, especially when talking about medical staff, work situations, or formal contexts.

You could also make it even more respectful toward the pharmacist with expressions like 説明してくださいました, but the original sentence is already perfectly natural and polite.