Learn Hardcore Japanese: kaeri ni hamigakiko wo kauno wo wasureru tokorodatta. - I almost forgot to buy toothpaste on the way home.

Breakdown of kaeri ni hamigakiko o kauno o wasureru tokorodatta.

Why does the sentence have two を particles?

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:

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

What is the の after 買う doing?

The の is nominalizing the verb phrase.

That means it turns:

into something like:

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

So:

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

Could this use こと instead of の?