Translation:The good children play in the squares, not in the streets or on the sidewalks.
So to clarify, the -en in this context means 'on the squares' in terms of literally making contact with the surface of the squares while they play? So would terekben mean 'in the squares' as in within the parameter of the square, maybe as someone looking down from above would see? I'm having trouble distinguishing these subtle changes in meaning in similar case endings.
Tér is one of the nouns that shorten their vowel when used with certain suffixes. Specifically -t (accusative), -k (plural), and any of the possessive suffixes. So you get:
- teret, terek, terem, tered, tere... , tereim, tereid, terei...
With most other suffixes attached, the vowel remains long:
- téren, térhez, térért, térnek, and so on.
There are plenty of words that do this, for example:
- bogár - bogarak - beetle
- név - a nevem - name
- kéz - kezet - hand
- levél - levelek - letter
Together with the words that lose a vowel (tükör - tükrök - mirror), the v-stem words (ló - lovak - horse) and the very few weirdos (teher - terhet - burden) they form the group of "stem-changing words".
Actually, the usage of "nor" in that suggested sentence is considered incorrect English.
"I drink neither coffee nor tea, but juice and water."
"I don't drink coffee or tea, but juice and water."
"That decision was neither good nor bad. It was just logical."
"The decision wasn't good or bad. It was just logical."
When we use "neither ... nor", we're connecting negative alternatives together. When we use "not", "or" is used to connect alternatives together. Because the negative sense of "not" carries through to everything listed after it, "nor" is redundant. The difference between "neither" and "not" is that "neither" only negates the first thing that comes after it and "not" negates everything that's listed after it. Think of "I drink neither coffee nor tea" more like "I drink" "not coffee" "not tea" and "I don't drink coffee or tea" as "I don't drink" "coffee or tea". Similarly, "That decision was neither good nor bad" can be thought of as "That decision was" "not good" "not bad" and "That decision wasn't good or bad" can be thought of as "That decision wasn't" "good or bad".
"Nor" can be used with "not" in a sentence but in a different sentence construction.
“Santa will not permit naughty behavior nor even consider bringing presents.” (https://www.quickanddirtytips.com/education/grammar/when-use-nor?page=1)
For other examples and more explanations:
The use of "nor" in your suggested sentence is generally considered incorrect English. For a decent article on when to use "nor", see https://www.quickanddirtytips.com/education/grammar/when-use-nor. My other comment lists more references.
"The good children play at the squares, not in the streets or on the sidewalks." (Reported 2019-08-26.)
Prepositional phrasing doesn't always translate one-to-one between any two languages. While Hungarian speakers may think of it as "on", English speakers may think of it as either "on", "in", or "at" (depending on dialects). While we're learning Hungarian, the biggest test is seeing if we get the Hungarian phrasing correct regardless of how we would (naturally) translate the Hungarian into English.