kaze wo hiita node, watasi ha yakkyoku de kusuri wo kaimasita.

Because 風邪をひく is the standard Japanese expression for to catch a cold.

In this expression:

• 風邪 = a cold
• を marks it as the object
• ひく = the verb used in this fixed phrase

So even though English says catch a cold, Japanese literally uses a verb pronounced ひく with 風邪. You should learn 風邪をひく as one set expression.

ひいた is the plain past form of ひく in 風邪をひく.

So:

• ひく = to catch a cold
• ひいた = caught a cold

It is past tense because the speaker first caught a cold, and then bought medicine.

In Japanese, subordinate clauses often use the plain form, even when the main sentence is polite.

So this is very normal:

• 風邪をひいたので、薬を買いました。

The first part uses plain style:

• ひいたので = because I caught a cold

The main clause uses polite style:

• 買いました = bought

This kind of mix is extremely common and natural. The politeness of the whole sentence is mainly carried by the final verb.

ので gives a reason or cause. In this sentence, it means because or so.

So:

• 風邪をひいたので = because I caught a cold / since I caught a cold

The sentence can be understood as:

• Because I caught a cold, I bought medicine at the pharmacy.
• I caught a cold, so I bought medicine at the pharmacy.

Both are good English translations.

Both ので and から can mean because, but they feel a little different.

• ので sounds a bit softer, more explanatory, and often more objective
• から can sound more direct or more like the speaker’s own reasoning

For example:

• 風邪をひいたので、薬を買いました。
  = softer, explanatory
• 風邪をひいたから、薬を買いました。
  = also correct, slightly more direct

In a sentence like this, ので sounds very natural.

は marks 私 as the topic of the sentence.

So:

• 私は = as for me / I

It tells us that the speaker is the person being talked about. In this sentence, 私は applies naturally to the whole situation: the speaker caught a cold and bought medicine.

Also, in Japanese, 私 is often omitted if it is already obvious. So this sentence could also be:

• 風邪をひいたので、薬局で薬を買いました。

That would still sound natural.

Yes, it could definitely be omitted.

Japanese often leaves out subjects and topics when they are clear from context. So both are natural:

• 風邪をひいたので、私は薬局で薬を買いました。
• 風邪をひいたので、薬局で薬を買いました。

Including 私は can make the sentence a little clearer, or it can add slight emphasis to I.

Because で marks the place where an action happens.

Here, the action is 買いました = bought, and that action happens at the pharmacy.

So:

• 薬局で = at the pharmacy

By contrast, に is often used for:

• destination
• time
• existence

Examples:

• 薬局に行きました。 = I went to the pharmacy.
• 薬局で薬を買いました。 = I bought medicine at the pharmacy.

So で is correct because it marks the location of the action buying.

They are different words:

• 薬局 = pharmacy / drugstore
• 薬 = medicine

So in this sentence:

• 薬局で = at the pharmacy
• 薬を買いました = bought medicine

Even though both contain the kanji 薬, they are not the same thing.

No, it is not strange in Japanese.

The sentence:

• 薬局で薬を買いました。

is completely natural. Japanese does not avoid this kind of repetition as strongly as English sometimes does.

If you wanted to be more specific, you could say something like:

• 薬局で風邪薬を買いました。
  = I bought cold medicine at the pharmacy.

But the original sentence is perfectly normal.

Because the speaker is describing a completed action.

The sequence is:

1. caught a cold
2. bought medicine

So both events are in the past:

• ひいた = caught
• 買いました = bought

If the speaker were talking about a habitual action or a future action, a different form would be used. But here, the sentence describes what happened.

Yes, Japanese word order is fairly flexible as long as the particles make the roles clear.

For example, these are all natural or understandable:

• 風邪をひいたので、私は薬局で薬を買いました。
• 私は風邪をひいたので、薬局で薬を買いました。

The most important thing is that the particles show what each word is doing:

• を marks the object
• は marks the topic
• で marks the place of the action

That said, putting the reason first with ので is very natural here.

The comma helps show a pause between the reason and the main action.

So:

• 風邪をひいたので、 = because I caught a cold, ...

In Japanese, commas are often used for readability and rhythm. The comma here is natural, but punctuation in Japanese is sometimes more flexible than in English.

It is read:

かぜ を ひいた ので、わたし は やっきょく で くすり を かいました。

Word by word:

• 風邪 = かぜ
• ひいた = ひいた
• 私 = わたし
• 薬局 = やっきょく
• 薬 = くすり
• 買いました = かいました

Yes. It is often written as:

• 風邪を引いた

In your sentence, it is written in kana as ひいた, which is also very common.

So both are natural:

• 風邪をひいた
• 風邪を引いた

Japanese often uses kana for common verbs or fixed expressions, especially when that makes the sentence easier to read.