Thanks for bringing the question, bardofages7604.
Thanks for answering the qestion, George...
I think George's explanation is good enough.
About 이/가, there are a lot (I mean a lot) to talk about...
It's not easy to explain about 이/가 clearly.
There are many usages for 이/가.
What's your level, bardofages7604?
If you are a Absolute beginner / Beginner, it's better for you to keep it simple.
Because the more I explain about 이/가, the more questions you could have in your brain. Simply... if the subject noun ends in an vowel, attach 가 to it. and if the subject noun ends in a consonant, attach 이 to it. That's definitely enough for you at your level.
If you are an Intermediate or higher level, then let me know.
I will give you some good explanation about 이/가.
BUT ONLY IF YOU ARE HIGHER LEVEL!!!
Even though many Korean use and speak 이/가, then don't know or can't explain about 이/가 clearly. As I said above, there are so many usages of 이/가.
George gave you some of them. But there are much more...
In my perspective, 이/가 can be a subject for each Absolute Beginner, Beginner, Intermediate, Advanced, and Higher Advanced level. Now you see my point!
I have to give you examples and explanation based on your level.
I hope you would come to understand of me...
cheers bardofages7604,
Tim