'Certain' in the Bible

Then the LORD told Abram, "You can be certain about this: Your descendants will be foreigners in a land that isn't theirs. They will be slaves there and will be oppressed for 400 years.

He reached a certain place and spent the night there, because the sun was setting. He found a stone there, used it for a pillow, and slept there for the night,

One of them left me, so I concluded "I'm certain that he has been torn to pieces," and I haven't seen him since then.