"That is it!"
Translation:To je ono!
"Je to ono?" could be said as in "Is that it?" But I've never heard it with the meaning "That is it!" even when talking to people from Moravia.
In Czech, you generally put the known facts at the beginning of the sentence (oversimplifying here!) and THAT is what you'd been talking about before saying this sentence, so to me, it only really makes sense at the beginning.
I must agree with you. I used to have problems with the "tak" (and sometimes I still have) 'cause it has many uses depending on the context. But the diference i've been told is in the use, when they achieve something or when they succesfully did something. Example: when i was moving in, some friends help me move mi forniture and when they put it in a place that they like it, they shouted: "Tak!"