General explanation: Russian nominative case is equivalent to English one, genitive case is equivalent to English possessive case. Examples: Этот торт очень вкусный - This cake is very tasty. (Nominative) Вкус этого торта напоминает мне моё детство - The taste of this cake (this cake's taste) reminds me of my childhood. (Genitive-Possessive). However, there are some differences: 1. The negation words like "нет", "не был", "не будет" etc. in the meaning "there is/was/were/will be" or "someone has/had/will have" require genitive case. У меня нет времени (gen.) - I have no time. 2. Some prepositions like "для" - for, "без" - without, "до" - before, "кроме" - except, "от" - from, "около" - next to, "вокруг" - around, "позади", "посреди" - in the middle of, "мимо" - by etc. require genitive case. Кроме тебя (gen.) все здесь. - Everybody is here except you. 3. Some verbs require genitive case - просить - ask (for), хотеть - want, желать - wish, требовать - require, искать - seek, look (for), ждать - wait (for), достигать - achieve, бояться - be afraid (of). Ты боишься собак (gen.)? - Are you afraid of dogs? Most of the Russian nouns with -a ending in nominative are feminine. Exceptions: the words that describe men - папа, дедушка.

I think you has genitive case of words. Торт and Сок -is nominative case. У меня нет торта и сока is genitive. Вouth nouns are male gender. the ending A do not index always on female gender. Te entendiste mi explicación?

Also, suffix "a" may mean nominative plural word

When you use genitive or ends some words.