Chuyển sang câu bị động bằng hai cách: They claimed that everything they said was true.  

Cách 1:
They claimed that everything they said was true. (active)
→ It was claimed by them that everything they said was true. (passive)

Cách 2:
They claimed that everything they said was true. (active)
→ That everything they said was true was claimed by them. (passive)

Everything they said was claimed as true.

It was claimed that everything they said was true by them.

Everything they said was claimed as true by them.

They claimed everything they said to be true.