kaeri ni hamigakiko wo kauno wo wasureru tokorodatta.

Because they mark two different objects in two different parts of the sentence:

• 歯みがき粉を買う = buy toothpaste
  • Here, 歯みがき粉 is the object of 買う.
• 歯みがき粉を買うのを忘れる = forget buying toothpaste / forget to buy toothpaste
  • Here, 歯みがき粉を買うの is turned into a noun-like phrase, and that whole phrase is the object of 忘れる.

So the structure is:

• [歯みがき粉を買う] の = the act of buying toothpaste
• [歯みがき粉を買うの] を忘れる = forget the act of buying toothpaste

This kind of double-を is very common in Japanese when one verb phrase is embedded inside another.

The の is nominalizing the verb phrase.

That means it turns:

• 歯みがき粉を買う = to buy toothpaste

into something like:

• 歯みがき粉を買うの = buying toothpaste / the act of buying toothpaste

That allows the whole phrase to become the object of 忘れる.

So:

• 買う = buy
• 買うの = buying / the act of buying

In English, we often use to buy or buying. Japanese often uses の for this kind of idea.

Yes, 歯みがき粉を買うことを忘れる is grammatically possible.

But there is a nuance:

• 〜の often sounds more natural and conversational for specific actions
• 〜こと can sound a bit more formal, abstract, or stiff in some contexts

So in everyday speech, 買うのを忘れる is very natural.

ところだった means something like:

• was just about to...
• almost...
• came close to...

So:

• 忘れるところだった = was just about to forget / almost forgot

In natural English, this whole sentence is usually expressed as:

• I almost forgot to buy toothpaste on the way home.

The important nuance is that the action did not fully happen. The speaker came close to forgetting, but remembered in time.

Because with 〜ところだった, Japanese usually uses the dictionary form of the verb to show an action that was about to happen but did not.

So:

• 忘れるところだった = I was on the verge of forgetting
• not I had already forgotten

If you used 忘れた, that would suggest the forgetting actually happened, which does not match the nuance of almost.

帰りに means:

• on the way back
• on the way home
• while returning

Here it most naturally means:

• on the way home
• or more literally, during the return trip

So the sentence is talking about buying toothpaste during the trip back.

Here, に marks the time/occasion at which something happens.

• 帰り = the return / the way back
• 帰りに = on the way back / when going back

This is similar to other time expressions with に, where it marks the moment or occasion of an action.

So:

• 帰りに買う = buy on the way back

It most naturally modifies 買う.

So the intended structure is:

• [帰りに [歯みがき粉を買う] の] を忘れるところだった

In other words:

• I almost forgot [to buy toothpaste on the way home].

It is not usually understood as:

• I almost forgot on the way home

Even though Japanese can leave some things flexible, the natural interpretation is that 帰りに goes with 買う.

That is very common.

歯みがき粉 can also be written as:

• 歯磨き粉

Both mean toothpaste.

Writers often mix kanji and hiragana for readability. In everyday Japanese, many words are written in partially kana forms even when a full-kanji version exists.

So:

• 歯 = tooth
• みがき / 磨き = polishing / brushing
• 粉 = powder

Historically the word literally means something like tooth-cleaning powder, though in modern Japanese it normally refers to toothpaste.

Very close, yes.

• 歯みがき粉を買うのを忘れるところだった
• 歯みがき粉を買い忘れるところだった

Both mean roughly:

• I almost forgot to buy toothpaste.

But the first version spells the idea out more explicitly:

• forget [the act of buying toothpaste]

The second uses the compound verb:

• 買い忘れる = forget to buy

Both are natural, though 買い忘れる is a bit more compact.

It describes a near miss.

The speaker is saying that they nearly forgot to buy toothpaste, but in the end they did remember.

So the implied situation is:

• they remembered at the last minute
• they did not actually end up forgetting

That is the key feeling of 〜ところだった here.