Prepositions and 'which' - how does it work?

Question

In English, you can get phrases, such as 'the house in which he lost his bag', but how would you do this in German?
I have made some attempts at a few, but I am unsure if this is how to do it, and if [not], how does it work?

• Ich habe den Teller verloren, von [dem/dessen] ich essen wollte! Er war auch so sauber!
  I lost the plate, from which I wanted to eat! It was so clean too!
• Er ist zu jenem Schwimmbad zurückgegangen, in dessen Nähe er seine Tasche verloren hat.
  He went back to that swimming pool near to which he lost his bag.
• Hier sind jene Kinder, mit [den/denen] er gesprochen hat.
  These are those kids that he talked to.

Answer 1

You are looking for relative clauses that begin with a preposition. As a first step you must know which case the preposition in question requires. Then you take the relative pronoun of the correct gender, number and case.
Take for example the sentence

Ich habe den Teller verloren, von ??? ich essen wollte!

As you can see in any dictionary of your choice, the preposition von requires the dative case. Then you must consider that Teller is a male noun in the singular number. So what you need is the Maskulinum-Singular-Dativ relative pronoun, which is dem or welchem. So the correct sentence is:

Ich habe den Teller verloren, von dem/welchem ich essen wollte!

OR: Ich habe den Teller, von dem/welchem ich essen wollte, verloren!

In your second sentence

Hier sind jene Kinder, mit ??? er gesprochen hat.

the preposition mit requires the dative case. Kinder is a plural noun, so you need the Plural-Dativ relative pronoun, which is denen or welchen. So the correct sentence is:

Hier sind jene Kinder, mit denen/welchen er gesprochen hat.

The third phrase is trickier because in dessen Nähe is not really a preposition:

Er ist zu jenem Schwimmbad zurückgegangen, in dessen Nähe er seine Tasche verloren hat.

This sentence is correct! A preposition that you could use here too would be nahe which requires the dative case. Schwimmbad is a neuter noun in the singular. So you need the Neutrum-Singular-Dativ relative pronoun, which is dem or welchem. Thus, another correct sentence would be:

Er ist zu jenem Schwimmbad zurückgegangen, nahe dem/welchem er seine Tasche verloren hat.

Answer 2

What I am about to say is not a scientific account based on data, but only meant as some rules that I can remember I use when formulating and interpreting relative clauses. Other people may use other rules and I do not have empirical data to claim that what I do is what most people do. After this warning, here is what I do.

First of all, it seems quite unnatural to me in German to mention a thing in terms of it being in a relatum of the location of an event as you put in your constructed examples. What I find more natural is to restrict a location with another location as in "komm zum Sofa, hier wo ich sitze!" or "kommst du mit zur Haupthalle, da wo alle sind?"

However, there are cases where such constructions may indeed be useful. Especially when explaining the relevance of something or someone to me, I think I would use structures like "das ist das Haus, in dem ich geboren bin" or more likely "das ist das Haus, wo ich geboren bin", and "das sind die Freunde, mit denen ich rechne". For locations, I would be more inclined to use "wo" than "in dem/der" whereas, for other complements and adjuncts, I would be more inclined to use a preposition.

Finally, events and unclassified physical things are mentioned in German typically by "das"/"da" and by "was"/"wo". Therefore, you should remember this when formulating and interpreting clauses such as "es hängt davon ab, was erwartet wird" and "ich habe das gefunden, wovon ich immer geträumt habe".

Answer 3

Roughly like you have it:

"Ich habe den Teller verloren, von dem..."

"Er ist zu jenem Schwimmbad zurückgegangen, in dessen Nähe..." (oder "wo er in der Nähe...")

"Hier sind die Kinder, mit denen er..."

You can also say something like this: "Ich habe die Hose verloren, die Du mir geschenkt hast, und in deren Tasche das Geld war, das ich mir geliehen hatte." --- "I lost the pants [which] you gave me. I had put the money [which] I borrowed into those pants ['s pocket]."