The most common Polish case used in everyday conversations?

which is the most common case used in everyday polish conversations? is there one specific one or are they all used to the same degree?

also, in a conversation...are all the cases mixed in a sentence or do you use one case for one sentence and then change to another case for your second sentence, depending on what you are trying to say.

Nominative

The nominative case is used in the dictionary form but I suppose the genitive case is also used a lot too.

I'd say Biernik (so Accusative), which has often a form identical with nominative (for masculin and neuter gender nouns), but it's only a guess

I'm afraid all cases can be mixed within one sentence, for example we can say: Marek (nominative) dał Gosi (dative) książkę (accusative) do angielskiego (genitive). (Marek gave Gosia an English handbook) - If it's what you meant in your question about cases.

what case would: spij slodko be then? i cant find which case changes spac (sleep) into this ending.

I'm still pretty new to learning Polish but I'm pretty sure 'spij' is the imperative form... it's not a case so to speak, but used whenever a direct 'command' is given e.g. poczekaj, sluchaj etc... however, someone with a little more authority on the matter may wish to correct me... :)

What happens if a direct object in a genetive or accusative sentence is plural?

Then decline the object to plural=)
Lubię dziewczynę - I like a girl(Accusative feminine singular)
Lubię dziewczyny - I like girls(Accusative feminine plural)
Nie lubię chłopca - I don't like a boy(Genitive masculine singular)
Nie lubię chłopców - I don't like boys(Genitive masculine plural)

So I keep it in the nominative plural (if that makes sense lol)?