Why not geldautomat ? Why end with
En at the end ?
yes, because “der Automat” is a noun of the so-called “n Deklination” (des Automaten, dem Automaten, den Automaten; plural: die Automaten etc)
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If there is einEN there also also should be -en at the end of a noun. It is a ‘der’ - word (der Geldautomat) and these words act that way. That’s why German nouns should be learned with articles.
ps! This explanation is as I see it. I’m no language expert. You can always ask in German forum. Mario knows better
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Hi Atikker,
sorry, it’s not quite as easy. Example: DER Staubsauger (vacuum cleaner), but “Ich habe einen Staubsauger”
I have no idea about the grammar in this case because as a native i just “speak” it.
Without the -en it sounds for me that you are looking for a specific ATM. Par example an ATM we would had talked about in a previous conversation.
With the +en it sounds that you are looking for any ATM. No a specific one. You just need cash and take every ATM you can get 
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that is not an explanation for the “Automat”… n-Deklination is a separate class (search google.de with “n deklination”, see or ex http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-N/Texte/Flexionskl.html?MenuId=Word111 or https://www.deutsch-als-fremdsprache-lernen.de/deutsche-substantive-deklinieren-faelle-kausus-deklinationsgruppe-n-deklination/). In cannoo or leo you can always see the Beugung/Deklination/inflection
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that a native would not know at all the grammar of their “own” language sounds strange… if i am not mistaken, some schools do teach grammar (at least in Othmarschen HH and some lost Gymnasien in Bayern???)
i guss i got u
english has the same rule
Well, this is the normal case.
See, a 6 year old child speaks perfect his/her native language but has no clue about grammar… magic? 
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