I can't figure out why the second sentence is composed the way it is..

(a guy visits a friend who is ill)
- "Hasta olduğumu biliyor muydun?"
- "Hayır, ev sahibin kapıyı açtığında söyledi."

I guess the sentence means "The house owner who opened the door told it". But what is "-da" doing at the end of açtığında?? Is this a situation where the locative should be used, like "when he was [standing there and] opening the door"?
I'm racking my brain here and I just can't see it..

I'll leave the grammatical explanations to the experts but:
"açtığında" means "when s/he opened"
so:
"ev sahibin kapıyı açtığında söyledi"=
your house owner said it when s/he opened the door

So, let me see if I understood it right..

kapıyı kapattığımda - when I closed the door
kapıyı kapattığında - when you closed the door
kapıyı kapattığında - when s/he closed the door
kapıyı kapattığımızda - when we closed the door
kapıyı kapattığınızda - when you closed the door
kapıyı kapattıklarında - when they closed the door

doğru mu?

YUP

Türkiye'ye gelince görüşÃ¼rüz.
Oraya gidince beni özle.
Konuşunca yüzünde güller açıyor.
Seni görünce içim bir hoş oluyor.
Yatağa girince hemen uyuyabiliyor musun?
Okuyunca anlıyor musun?
Sorunca cevap ver.

-ce is similar to -de.
*But I feel 'after' in most cases.

First of all thanks caliptrix pointing out the correct suffix.
not -ce. it is -ince.

If you haven't studied yet, I don't recommend you to read what I wrote below. It may confuse you more. I think it is out of my capabilities.
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1-)Türkiye'de yaz okuluna gittiğinde çok enteresan şey öğreneceksin.
2-)Türkiye'de yaz okuluna gittiğin zaman çok enteresan şey öğreneceksin.
3-)Türkiye'de yaz okuluna gidiyor olduğunda çok enteresan şey öğreneceksin.
4-)Türkiye'de yaz okuluna gidiyor olduğun zaman çok enteresan şey öğreneceksin.
5-)Türkiye'de yaz okuluna gidince çok enteresan şey öğreneceksin.
******
I am not sure how to explain. They all mean similar.
And differences between them are not very clear.
The things I wrote down are my ideas only, individually.
*I strongly suggest you to read a grammar book or wait for others' answers, before taking what I have written into your account.
******

1 and 2 : At the time when you attend the school.
3 and 4 : When you keep attending the school.
5 : After you keep attending/completing the school.

a)Kapıyı açtığımda Ali'yi gördüm.
b)Kapıyı açtığım zaman Ali'yi gördüm.
c)Kapıyı açınca Ali'yi gördüm.

a) at the time of 'doing' the action. Not after completing the action.
b) Can be used instead of a and b.
c) After completing the action of opening.

I)Türkiye'ye geldiğinde görüşÃ¼rüz.
II)Türkiye'ye geldiğin zaman görüşÃ¼rüz.
III)Türkiye'ye gelince görüşÃ¼rüz.
Here they all look same.

First of all, thanks a lot for all you help

There is something I've seen in this thread, and I saw it in another one as well:

I don't know why but, Turkish doesn't allow some constructions in some tenses. So we use 'olmak' to make them.

2-)Türkiye'de yaz okuluna gittiğin zaman çok enteresan şey öğreneceksin.
4-)Türkiye'de yaz okuluna gidiyor olduğun zaman çok enteresan şey öğreneceksin.

1 and 2 : At the time when you attend the school.
3 and 4 : When you keep attending the school.

here, we cannot say gidiyor-de or gidecek-de using -de suffix.
gidiyor olduğun(da) zaman.
The tense of the sentence(4) is Future. As it is same in English we cannot say future and continuous in the same verb(I think).
Look at the translation : you will 'be' attending.
We have to use 'be' to give a continuous meaning in the future.
(4)You will learn very interesting things while you will 'be'/'are' attending a summer school in Turkey.
Olmak is 'to be'.

(2)You will learn very interesting things when you attend a summer school in Turkey.

I can make more explanations later, if you want. I am late to my evening class. Gotta go..

Ah! Sorry guys. I was late that day and was in hurry.

It was a mistake. But thanks for correcting.
It should have been 'çok enteresan şeyler'
And I don't why I have used 'enteresan' it is not a Turkish word.

Çok ilginç şeyler.

And about 'gidiyor olduğun zaman'.
It is a correct and perfect Turkish sentence. Everybody uses it when it is required to say such sentence. I mean if you are trying to ''strictly emphasize a continuous'' action.
Though it is kind of interchangeable,
Gittiğinde doesn't give the continuity clearly, right ?

But this was not the point in my explanation above.
It was an example for the given sentence in previous posts.
I was trying to explain -de cannot be used with -yor tense.

So we have to add 'olduğun' to say -de.
And I tried to compare it with 'be' in English.

But, really thanks for the correction.
Take care,

Bir şey değil, Elisa.