to form an opinion about sth without knowing all the details or facts 推测；猜测；推断
~ (about/on/as to sth)
We all speculated about the reasons for her resignation.
我们大家都推测过她辞职的原因。
~ why, how, etc.…
It is useless to speculate why he did it.
对他为什么这么做妄加猜测毫无用处。
~ that…
We can speculate that the stone circles were used in some sort of pagan ceremony.
我们可以推测，这些石头排成的圆圈是用于某种异教崇拜仪式的。