"I have time."
It actually means "to have time" or "there is time". Of course, the meaning of the sentence depends on the context.
I think a more natural answer is "ひまです", which translates to "I am free/have time for you".
it's changing the topic or is the topic of the conversation so it's が. を makes it the subject but smaller, basically like talking about a party would be が and talking a little about the cake, presents, etc would be を in one sentence and not the whole conversation. (I could be wrong! ^^')
を is the object marker. It's が because the verb means 'to exist', so the sentence is literally "There is time", time being the subject.
Why is the 間 there? Why cant it just be 時? I never picked up on this, but i do know that 間 means between (on its own i think).
間 is also used to tell that you mean a specific amount of something, that it has a beginnig and an ending. You could rather see it like a gap. For example when you sit between to people, the beginning is the arm of one person, then there's a gap where you sit and then there's the next person. When talking about time, you don't have time in general (you still have something to do the next day, probably), but you have a specific amount of time, from the moment you're speaking until you have to sleep or whatever. Your free time is here like the gap between two things you have to do. That's why you use 時間, a time span (with 間, a gap).
I hope that's understandable (and true ^^" )
I liked how someone said think of 間 as amount. So you have time, a certain amount of time.
By mistake, I typed "時間あります" instead of "時間があります". And it showed correct. That means が is optional here, but why?
I just started learning this yesterday and jumped in without reviewing my notes. It was absolutely hilarious how much I froze and stared at this. XD Before realizing the correct answer, here were my two "I have no clue what I'm doing" answers: Tokei wa watashino desu! (The clock is mine!) Jikan masu! (Doing time!) (Exclamation points included because I was amused and panicking at the same time. I thought of "Kore wa watachino jikan desu." too, though, which is just "This is my time.") Eventually, I did realize the right answer. XD