I saw certain sentences in the first 15 chapters of GM that I want to compare with similar combinations. The first sentence of every item on the list is the one written in GM and the other one is a little bit changed to examine the difference that it makes in meaning:

1- mi promenis "de" starto al strato...
mi promenis "La" strato al strato...

2- Se mi estus trovigxinta naz-al-naze kun li gxuste kiam li eliris el la posxt-oficejo
Se mi estus trovigxinta naz-al-naze kun li "gxuste tiam" li eliris el la posxt-oficejo

3- Sur la maldekstra flanko de la strato, kiu iras al la cxefa Sportocentro
Sur la maldekstra flanko de la strato, kiu iras al la cxefan Sportocentron

4- li eniris restoracion
li eniris en restoracion

5- Mi miras pri la grava loko, "kiun" konsideroj pri manĝo kaj trinko okupas en via spionraporto.

Mi miras pri la grava loko, "kies" konsideroj pri manĝo kaj
trinko okupas en via spionraporto.

Mi miras pri la grava loko, "kien" konsideroj pri manĝo kaj
trinko okupas en via spionraporto.

(concerning the 5th one, I have to add that I don't fully get the -n thing at the end of kiu)

The difference in meaning is that most of your changed versions are grammatically incorrect, so they don't make much sense.

1) You have no preposition in your version. "from street to street" (= from one street to another street) makes sense in English but only because we have "from". If you say "the street to street" it doesn't make sense in English, and also not in Esperanto.

2) "If I were to find myself nose to nose with him, just [at that time] when he left the post office." We are talking about a hypothetical, so we use "kiam". It's a shortened version of saying "tiam, kiam" (just at that time, when; ĝuste tiam, kiam...). You can't say "ĝuste tiam, li eliris" because it means "Just then, he left". It doesn't match the first part of the sentence. The first part of the sentence is -us, so it's conditional; the second part is telling what would happen, if that condition were true. So we use "kiam". "Tiam" all alone doesn't make sense.

3) No -n after "al". Ever. -n after a preposition shows movement. So prepositions that show movement by definition (like "al" and "el") can never be followed by -n.

4. Here is the one case where your version is also correct. The difference in meaning is negligible. The original sentence says "He entered a restaurant" and your version says "He entered into a restaurant"

5. Your versions don't make a lot of sense. The sentence says "I wonder about the important place, that considerations about food and drink occupy in your spy report" In other words, food and drink occupy [a place] in the report. Subject: Food and drink. Verb: occupy. Object: a place. (hence -n ending). We use "kiun" to replace "important place" instead of mentioning it a second time in the sentence.

"I wonder about the important place; food and drink occupy [an important place = kiun] in your report".

That's why we need -n. We use "kiun" how I used "that" in the English version.

The sentence no longer makes sense if you substitute kies or kien.

Original sentence (with kiun):
I wonder about the important place, that [kiun] considerations of food and drink occupy in your spy report.

kies version: I wonder about the important place, whose considerations of food and drink occupy in your spy report.

kien version: I wonder about the important place, to where [kien] considerations of food and drink occupy in your spy report.

First off, thank you so very much for taking your time to write a response, I appreciate that...

1) You have no preposition in your version. "from street to street" (= from one street to another street) makes sense in English but only because we have "from". If you say "the street to street" it doesn't make sense in English, and also not in Esperanto.

It's perfectly O.K in Persian to say so, and it is the only correct way.

In Persian: خیابان به خیابان به دنبال او گشتم

(Literally: I searched for him, street to street)

about the last one, unfortunately it still sounds weird and ambiguous to me.

No -n after "al". Ever. -n after a preposition shows movement. So prepositions that show movement by definition (like "al" and "el") can never be followed by -n.

agreed. Though I may take a little issue with this reasoning:

a) veni al
b) alveni al

al shows direction (the way n does) and is repeated after alveni without being considered superfluous.

Yeah I know this case is a bit different to what you are alluding to, but it nonetheless appears to be a repetition

About the last one, if you can correctly use "that" and "who/which" in English, there's no reason why you shouldn't be able to use kiu/kiun in Esperanto. It's very analogous.

Mi ŝatas la libron, KIUN vi legas.
I like the book that you are reading.
(that = the book. You are reading the book, so I use "kiun", with -n, since it's the object)

Li vidis la viron, kiu venis.
He saw the man who came.

-n on "viro" because it's the object of "vidis". But no -n on "kiu", because although kiu is repeating "viro", it's the subject of "venis".
This is our way of making a combined sentence out of what would otherwise take two sentences.
He saw the man. The man came.
Li vidis la viron. La viro venis.

He saw the man who came.
Li vidis la viron kiu venis.

It's the same principle as the sentence from Gerda, only the sentence from Gerda is much more complicated, so it makes the grammar seem more difficult than it really is.

omid17:Though I may take a little issue with this reasoning:

a) veni al
b) alveni al

al shows direction (the way n does) and is repeated after alveni without being considered superfluous.

Yeah I know this case is a bit different to what you are alluding to, but it nonetheless appears to be a repetition

This is a different type of repetition, although good observation

When you repeat -n after al, you're basically saying "to towards", which I'm guessing for many people including me, to try and understand what that means would cause our brains to explode

As for "alveni", what the "al" is doing here is really becoming an adjective instead of a preposition (al alone) or an al-stand-in (-n after a preposition). So "alveni" really means "to come in a to-ish way".

Whether you choose to use it is up to you, since I personally don't see it necessary other than to make the word sound different, so I myself will try to avoid it.

Translate "alveni" as "arrive".

alveni al = to arrive "to" [a destination]

In English we would say arrive AT, but in Esperanto we say "arrive to".

alveni is a more directional word than veni. You left with a specific destination in mind and you have arrived at your planned destination. Veni is a bit more neutral. You didn't have to have a destination in mind when you came, for "veni".