1.- se dice que tu amigo comprará una tarta a su prima
(activa) they say (that) your friend will buy a cake for his cousin.
(pasiva) IT IS SAID (that) your friend will buy a cake for his cousin.
(pasiva) Your friend is said to buy a cake for his cousin
they think that the next exam will be difficult.
IT IS THOUGHT that the next exam will be difficult.
The next exam is thought to be difficult
they believe (that) the books are not safe here
IT IS BELIEVED (that) the books are not safe here
The books are believed not to be safe here.
4.- dicen que es tarde.
They say (that) it is late.
IT IS SAID TO BE LATE
they think/believe that you will pass
IT IS THOUGHT/believed (that) you will pass
(you are thought to pass)