Question

think 究竟加 to do sth 还是 doing sth。不是think of

Answer 1

think to do sth. 想要,打算[+to-v][+(that)]
At first he thought to tell her, then he changed his mind.
他原来打算告诉她的,后来改变主意了。
think 后面经常跟从句。
think 后面要是加doing sth.时，总是先加介词，再加doing sth.

Answer 2

think sb to do sth.