'Certain' in the Bible

Then the Lord said to Abram, "Know for certain that your descendants will be strangers in a foreign country. They will be enslaved and oppressed for four hundred years.

Then Isaac said to Jacob, "Come closer so I can touch you, my son, and know for certain if you really are my son Esau."

He reached a certain place where he decided to camp because the sun had gone down. He took one of the stones and placed it near his head. Then he fell asleep in that place