Using 時 to Express "When..."

I know I've used 時 clauses in the past for various "when..." expressions, but there are some important rules to keep in mind when using it with verbs. Genki's explanation for this grammar point is rather convoluted, frankly, so I've consulted some other sources to help me better understand it.

To express "when..." in Japanese, the following sentence pattern is used:

Sentence A 時, Sentence B.

It means "When A, B." Sentence A always ends in the plain form, in the present or past tense. Here is a StackExchange example that has helped me better understand this. Quoted from that link:

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

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

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.

I actually just had a bit of a breakthrough in thinking about one of Genki's example sentences, so I'm going to write that here and hope it helps this grammar point stick.

中国に行った時、ウーロン茶を買います。
I will buy oolong tea when I go to China.

At first, it seems strange that it's 行った rather than 行く. But the English translation is misleading. Because we're not buying the tea until after we've gone to China, we need that past tense. So perhaps a more accurate translation is:

中国に行った時、ウーロン茶を買います。
I will buy oolong tea when I've gone to China.

This is much easier to understand, and clearly lets us know that you're not going to buy the tea on your way to China, but rather, that it will be bought once the "going" is complete and you're already there. Hurray for breakthroughs!

If a noun is used directly before 時, then it must be followed by の. Na-adjectives must be followed by な. I-adjectives don't require anything special.

元気な時、公園を走ります。
I jog in the park when I feel fit.

犬が病気の時、病院に連れていきました。
I took the dog to the vet when it was sick.

さびしい時、友達に電話します。
I call up friends when I am lonely.

All of those are Genki examples. Note that the adjectives and nouns do not need to be conjugated for tense. They're fine just the way they are.

This is probably one of the most difficult grammar points I've studied thus far, because it's so completely different from English. It definitely calls for more reading practice to get the hang of it.