I can = I am able to/I have the ability to. I could = I am able to/I have the ability to AND I'm not sure if I will. It's expressing uncertainty (like when you have options). But, in this case it's more of a tense issue. Can't is present tense and couldn't is past tense. "I can't eat until dad gets home" is talking about the present, but "I couldn't eat until dad gets home" seems like it's missing something. "Yesterday, I couldn't eat until dad got home" or "Tonight, I couldn't eat until dad got home" sounds more correct.

It is actually a bit confusing but I will try to explain it. Could is used as explained but it is also used as a polite form of can as in: "Could you please pass me a plate?" OR as the past tense of "can": "I could not have imagined a worse outcome" It is not used as an impossibility in the present tense. For example, in the present tense you would say: "You can not be serious!" but if you were speaking in the past tense, you might say "He/She/They could not have been serious!". So in your sentence you could say " I couldn't eat" but it's not the same as saying "I can't eat until Dad comes homes" because one "couldn't" indicates past tense in this instance while "can't" is present tense. Hope this helped!

In this case both are correct. In my opinion "I couldn't eat" significates lack of appetite whereas "can't eat until my dad comes home" means maybe there isn't any food or the person can't cook before the dad comes home. Can expresses ability yes. Example: can you please help me? Can you take out the trash ? And could is an option; for example "I could go to the supermarket before I go home"

You need to study the grammar of "can" and "could" in more detail because your summary is too simplistic to be useful. "can" - use for possibility and ability, both in the present (going into the future). The context will tell the listener which sense is being expressed. "could" - use for past ability and for present (doubtful) possibility. You would need to practise these usages for some time to become confident with them.

"Can" may be used to express both ability and possibility. Ability: "I can play the guitar." Possibility: "Learning a language can be difficult" "Could" is the past conjugation of "can". It also may be used to express ability and possibility. Ability: "When I was a child, I could speak Japanese" Possibility "I could try to study English" In your sentence, the speaker is referring to the present, so he will use "can". If he was speaking about the past, "could" would be correct: "Yesterday, I couldn't eat till Dad got home." That are maaaaaany usages of "can" and "could", but I hope this helped you understand it a little better.