The use of present and past tenses in ～時 sentences


In Genki II textbook (second edition page 114 and 115) you are tasked with
creating sentences using ～時. The problem is their answers seem to contradict
what is written in "A dictionary of basic Japanese grammar" (page 493).

Ex IV.A.2 shows a picture of a student not understanding something, we are
supposed to create a sentence meaning "When I don't understand something I ask
someone". So the order seems to be I don't understand something , and then I
ask someone, which according to dictionary asks for past + 時 + present
(わからなかった時人にききます), while the answer is present + 時 + present (わからない時人にききます).

Similarly, ex. IV.A.4 shows a person being homesick. We are supposed to
construct a sentence meaning "When I get homesick, I call my parents", which
again seems to call for past + 時 + present, and instead it's translated as
ホームシックの時両親に電話をかけます.

I'm pretty sure I'm misunderstanding something.




The past-tense ～た時 pattern is used when the action is **completed** relative
to the main clause.

The present-tense ～る時 pattern is used when the action has **not yet been
completed** relative to the main clause.

A common example to illustrate the difference:

> 日本に行くとき、カメラを買った。  
>  _On the way_ to Japan, I bought a camera. (The action of "going" isn't
> complete yet.)  
>  日本に行ったとき、カメラを買った。  
>  _When I went_ to Japan (after arriving), I bought a camera.

This is reflected in your sentences as well:

> When I don't understand something I ask someone.  
>  わからないときは人に聞きます。

Your lack of understanding is _not yet completed_ when you ask. It would not
make sense to ask once you understand. Therefore, the present tense is
necessary before 時.

Likewise, you call your parents while being homesick, not after being
homesick. So the present tense is used in that sentence also.




The past-tense ～た時 pattern is used when the action is **completed** relative
to the main clause.

The present-tense ～る時 pattern is used when the action has **not yet been
completed** relative to the main clause.

A common example to illustrate the difference:

> 日本に行くとき、カメラを買った。  
>  _On the way_ to Japan, I bought a camera. (The action of "going" isn't
> complete yet.)  
>  日本に行ったとき、カメラを買った。  
>  _When I went_ to Japan (after arriving), I bought a camera.

This is reflected in your sentences as well:

> When I don't understand something I ask someone.  
>  わからないときは人に聞きます。

Your lack of understanding is _not yet completed_ when you ask. It would not
make sense to ask once you understand. Therefore, the present tense is
necessary before 時.

Likewise, you call your parents while being homesick, not after being
homesick. So the present tense is used in that sentence also.

