According to the book I'm reading, both of these sentences translate to: “I
bought a bag when I went back to my country”.

  1. 国へ帰るとき、かばんを買いました。
  2. 国へ帰ったとき、かばんを買いました。

Now the nuance, if I understand correctly, is that

  * #1 is saying that "on the way" to the country, she bought a bag. In other words, she bought the bag en route but before she arrived at her country. 
  * #2 is she arrived at her country and then bought the bag.

Is my understanding correct?
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2013-04-12T07:41:39.847
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question
verbs phrases tense
What's the difference between “plain form + とき” and “ta-form + とき“?
I do not think the nuance you listed gives the full picture.

  1. 国へ帰るとき、かばんを買いました。

This can be said if you bought the bag before you returned (i.e., on the
airplane, in the departure airport, etc.).

  2. 国へ帰ったとき、かばんを買いました。

This can be said if you bought the bag before you returned, or when you
completed returning.

So, the natural reading of the second sentence is probably that the bag was
bought when you returned, but it's rather ambiguous and could easily mean
before you returned.

* * *

There is a more important difference between the sentences though, which is
that the former sentence does not necessarily imply that you got home:

> 国へ帰る時、かばんを買いました。でもやっぱり帰りませんでした。  
>  "When I was returning to my country, I bought a bag. But, it turns out that
> I never actually got home."
>
> 国へ帰った時、かばんを買いました。でもやっぱり帰りませんでした。  
>  Semantically invalid, 帰った conflicts with 帰りませんでした.

As mentioned, your first sentence does not necessarily imply that you got
home, which is why you can add でもやっぱり帰りませんでした.

* * *

Note, that behavior of the plain form in relative clauses only works like that
for a state-change verb (e.g., 帰る), not for action verbs (e.g., 走る).

> 道を（毎日）走る時、宇宙人を見たことがありました。 "Back when I was running the streets (every day),
> I saw an alien."

When your embedded verb (走る) is an action verb in plain form, if the matrix
predicate (ある) is past-tense, the embedded verb is forced to take on a
habitual reading.

In this case adding でもやっぱり走りませんでした, "but I never ran" does not make sense
semantically.
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answer
I'm Japanese. I hope to improve my English and use English more often, so I'll
answer your question.

As Darius-san wrote, 2 is ambiguous, and most Japanese think that she arrived
in her country and then bought the bag. But if I translate these sentences
without thinking well, I might do both to "I bought a bag when I went back to
my country." Given this, I come to think that this English sentence is also
ambiguous. When do you think the speaker bought a bag?

By the way, the sentence "国へ帰る時、かばんを買いました。でもやっぱり帰りませんでした" Darius-san wrote is
pretty strange. Japanese never say so.

If you say this, it is correct:

> 国に帰るためにかばんを買いました。でも、やっぱり帰りませんでした。  
>  I bought a bag to return to my country. But it turns out that I never
> actually got home.

Your first sentence necessarily implies that she got home.

My English may prevent you from understanding. Sorry.
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answer