When talking about certain states, the past tense is often used. There is also a difference between English and Japanese in how we describe hungriness. In English, we say "I am hungry," which describes your feeling, rather than the physical state of your stomach as is done in Japanese by saying お腹がすいた (My stomach is empty (emptied)). Also, keep in mind that you could say お腹がすいている (lit: My stomach is emptying), which describes the on-going state of becoming hungrier and hungrier. You would never say お腹がすく, because the present tense in Japanese doesn't exactly express the present moment. It usually expresses actions that will happen or that occur by habit or with some sort of scheduled regularity. So, you are limited to how you can express hunger. お腹がすく "I am going to be hungry." OR "I am hungry (at this particular time; for example: いつも9時ぐらいお腹がすきます = I always get hungry around 9 o`clock) お腹がすいています "I am hungry (and have been for a while)" お腹がすいた " I am hungry" Does this make sense? If you have any questions, please let me know.

I'm not sure the diffrence between おなかがすいた and おなかがすいています exactly, but if someone says おなかがすいています to me, i feel like he/she wants to eat NOW. "います" sounds like he/she is emphasising the present tense =NOW. So i would say おなかがすいた sounds more natural, but you can use おなかがすいています if you are really hungry and want to eat sth right now. (by the way, i guess おなかがすいた is not the past tense but present perfect)

I have never thought about it. Does this help? http://www.guidetojapanese.org/blog/2005/07/13/being-hungry-or-full/

Thank you, everyone, for the answers. You all have helped me.