"You are a child."
Jsi and Ty jsi are the same. The subject "Ty" is often omited because the meaning is obvious without it.
Same problem. Can I always write "Jsi" without "Ty"? Are there any exceptions?
Yes, there are. You will meet some in the course. Where you are directly referring to YOU, ypu will need it.
When you stress "ty" by adding it here, it should go to the first position. And "jsi" to the second unstressed position. "Jsi" is sometimes clitic, so it is "at home" in the second position.