firstly;if i wanted to say in the passive,''tonight fish and grapes and apples were eaten by us''what gender should the verb go in,when there are three nouns of different genders and number in the sentence,like this''hac nocte piscis et uvae et mala nobis -is it comesi, comesae,comesa sunt,''from the verb comedo comedere.
secondly;which case do i put the noun in when the verb is intransitive,i usually put it in the nominative case,is that correct?
if you wanted to literally translate the passive sentence "Tonight fish and grapes and apples were eaten by us", you should say correctly : "Hac nocte pisces et uvae et mala a nobis comesi sunt” where the past tense, passive voice, “comesi sunt” is in the nominative masculine plural, because the plural masculine noun “pisces” is a living being and therefore predominates over the nonliving things “uvae” (feminine plural) and “mala” (neuter plural).
Anyway, you could also say: "Hac nocte pisces et uvae et mala a nobis comesa sunt “ where the past tense, passive voice, “comesa sunt” is in the nominative neuter plural, because when nouns of different genders include both living beings and things without life, the predicate adjective or the passive compound verb sometimes agree in gender with the nearest, i.e. “mala” (nominative neuter plural) in this context.
As for the nominative singular “piscis” and the ablative plural “nobis” that you used in your translation, please note that they are not correct because:
1)the singular “piscis” means “a fish”, not “fish” in the plural, apart from poetry where you can find “piscis” instead of “pisces”.2) the ablative plural “nobis” needs the preposition “a” as this is an “Ablative of Voluntary Agent” (with a/ab after Passives verbs).
Lastly, with regard to the case you must use when the verb is intransitive, I think that you mean that the subject of an intransitive verb as well as of a transitive verb is in the nominative case, of course.
For example:”Sol lucet” ( the sun shines / is shining);”Caesar profectus sum” (Caesar left), where the subject is obviously in the nominative case and there is no direct object as an Intransitive Verb admits no direct object to complete its sense, while a Transitive Verb has or requires a direct object to complete its sense as in “Caesar classem hostium profligavit” (Caesar dispersed the enemy's fleet).
As you can see, both Transitive (profligavit) and Intransitive verbs (lucet, profectus sum) have the subject in the nominative.