"...to iu koto desu。” we use in daily conversation "○○○to iu koto ka。" we use this sentence when we are convinced of ○○○ Kare wa sensei desu. (Explanation) Kare wa sensei to iu koto desu. (Explanation more emphatic) Kare wa sensei to iu koto ka. (Explanation even more emphatic) →If we use this sentence, speaker's feeling is「Oh he is teacher.」「Ok I see I understand he is teacher」 So I think that when using "Past Toki....Future" I can't use this, →Yes. if we talk about some action that takes place for sure. I saw this sentence in a card game and they use "Past Toki...Future \ Present): Kono kaado ga shoukan ni seiko shita toki, subete no kaado wo hakai suru. This case [toki] express about “condition” [Kono kaado ga shoukan ni seiko shita toki] →If (toki) this card (Kono kaado) succeed (seiko shita) shoukan, someone(something) will be able to deatroy all cards (subete no kaado) . *Sorry [shoukan]=I didn’t understand The example is future sentence. Yami to yami kasanatta toki, meifu no tobira wa hirakareru. This case is the same. Nihon ni iku toki, o-miyage wo kaimasu. (Before arrive to Japan) →If you want to say this sentence before arrive to Japan You can say Nihon ni ittara, o-miyage wo kaimasu. When I go to Japan I will buy some souvenirs. →If you want to say this sentence, when you arrived in Japan Nihon ni tuita node, o-miyage wo kaimasu. I have just arrive in Japan and I am buying some souvenirs However I think [Nihon ni tuita node, o-miyage wo kaimasu.] is just strange. Because I cannot buy souvenir as soon as arrive to other countries. There are so complected. Sorry If my explain was short.

shita toki = the point in time just after you have done it This can refer to the past or future. suru toki = the point in time just before you do it Again, can refer to the past or future. Nihon ni iku toki, o-miyage wo kaimasu. (When I GOJapan)...e.g at the airport departures Nihon ni ittara, o-miyage wo kaimasu. (When I GO to Japan)...this one is not specific about the point in time, although I suspect it means "When you are in Japan". When I arrive in Japan = Nihon ni tuita toki. Another practical way of saying it: Nihon ni itte, omiyage wo kaimasu. The bit about "to iu koto" you have already asked, and it has been answered. Why do you ask again?