"Buy ice cream for dessert."
Translation:Купи мороженое на десерт.
My understanding is that while both для & на can mean "for", для десерта means "for dessert" in the sense of "to improve the dessert", whereas на десерт means "for dessert" in the sense of "so that it can be the dessert". Subtle differences and both are possible interpretations of "for dessert" in English, but the latter is far more likely in this context.
Yes, if you use для, you indicate that you're talking about an ingredient to go into the dessert, e.g. "buy sugar for the cake" or "carrots for the soup".
If you intend to say "we're going to have X for dessert/dinner/lunch/breakfast/supper, then the set expression is на + akkusative.
Well, the meaning wouldn't be exactly the same...I would interpret your text as you were going to make something for dessert where the ice cream is going to be an ingredient. The English text could mean that, so it's not entirely wrong, but I think the idea here is that they're going to HAVE ice cream for dessert, rather than USE it to make a dessert... :-)
Для means that an object is directed to somebody or something: подарок для меня, колесо для машины, земля для крестьян, заводы для рабочих, цветы для женщин, мороженное для детей. In most cases it can be substituted with Dative without prepostion: подарок мне, мороженное детям, etc. За (in your case) means the reason: спасибо за помощь (I'm thanking you because you helped me), награда за храбрость (you are awarded because you were brave), плата за работу (I pay because you worked), etc. "For time" construction is translated usually with на: на 5 дней.
Купи=buy it (once). Покупай=buy it (everytime you see it). For example, I really love one kind of tea, but in 50% cases there is no such kind of tea in local supermarket, so my family has a rule "покупАй Этот чай всЯкий раз когдА егО вИдишь" - "buy this tea every time (when) you see it". But if I want an ice cream for пОлдник (dessert time between lunch and dinner, something like British 5 o'clock tea), I'll call my boyfriend and say "купИ морОженое, когдА бУдешь идтИ с рабОты" - "buy an ice cream when you will be walking from work".
I haven't gotten to the adj.spell module yet, but it appears that Мороженое is not invariable/indeclinable, so, although it acts as a neuter noun, its endings regarding case and number are determined by the rules for spelling adjective endings?
And - is it ever plural? In American English, we would never say "ice creams", but rather a phrase such as "6 kinds of ice cream" or "6 pieces of ice cream (the kind on a wooden stick)".
Come to think of it, you option is also correct.
It is just that if you mean X is supposed to be served as a breakfast / lunch/dinner/"1st course"/"2nd course"/dessert you say that X is на + the name of the meal (Accusative but it hardly matters).
The subtle difference is, in the latter case ice-cream does not exactly help you eat or make your dessert better—it IS the dessert.