没有上下文,不能确定时态,现在时话是I find Mr.Jack sitting and reading under the tree.
如果是过去时I found Mr.Jack sitting and reading under the tree.
这里应该是考find 的用法吧
find sb do sth是发现某人做某事的全过程.= sb be found to do sth
find sb doing sth是发现某人正在做某事,强调正在进行之中.