Traduzione:Du fährst kein Ski.
Mi sono chiesto perché ci fosse "...kein Ski" e non "...keinen Ski" visto che Ski è maschile. Dopo una piccola ricerca ho trovato una bella risposta in Duolingo nel corso di tedesco per chi parla inglese, da parte di Wataya. Pensando che possa servire ad altri mi permetto di ritrascrivere qui di seguito. E' in inglese, ovviamente, ma mi sembra abbastanza comprensivo.
"Let me give it a shot..The two most important things to say about this sentence are
-Duo's solution is correct
-You won't be able to reproduce it by following the rules in your grammar book.
If you want to negate the sentence "Du fährst Ski", you have two possibilities: you can either negate the verb or the noun. In the first case you arrive at "Du fährst nicht Ski". This is totally fine and idiomatic and probably the easiest solution here.
If you instead choose to negate the noun you have to be aware that "Ski" can be either singular "der Ski" or plural "die Ski" here.
So, following the rules in your grammar book you arrive at
-Du fährst keinen Ski (singular)
-Du fährst keine Ski (plural)
The latter solution is rarely used, the first one (which really sounds weird to native ears if used in this meaning) virtually never. This is one of the cases where the grammar rules fail to reproduce the actual usage (think of "take it easy" vs "take it easily" in English). The most common phrasing of the sentence is "Du fährst kein Ski". I suspect this to be formed analogically to "Du fährst kein Auto" and "Du fährst kein Rad" (in these two cases the grammar rules give you the right result). I am not completely sure whether the Duden would consider "Du fährst kein Ski" part of the standard language or just a very common regionalism. For practical purposes it doesn't really matter. It's very common and idiomatic. So just use it.
BTW: "Ski fahren" is not a separable verb but "noun + verb". Note that the corresponding noun "das Skifahren" is written as one word but if you use it as a verb the two words have to be written separately: "Ich kann nicht Ski fahren". http://www.duden.de/sprachwissen/rechtschreibregeln/getrennt-und-zusammenschreibung#K54
Credo che anche "Du fährst nicht Ski" sia accettabile. "Ski" non intende l'oggetto "lo sci", ma "Ski fahren" si capisce come verbo frasale, che può venire negato con "nicht".