Rashi on Genesis 18:19/רש"י על בראשית י״ח:י״ט

1כי ידעתיו FOR I KNOW HIM — this is an expression denoting "affection", as (Ruth 2:1) "kinsman (מודע) of her husband," and (Ruth 3:2) “And is not Boaz our kinsman (מודעתנו), and (Exodus 30:17) “I have distinguished (ידעתיך) thee by name”. Still the primary meaning of these terms connected with the root ידע is really that of knowing, for whoever holds a person in affection attaches him to himself, so that he knows him well and is familiar with him. And why do I hold him dear?

2למען אשר יצוה BECAUSE HE WILL GIVE HIS CHILDREN CHARGE CONCERNING ME TO KEEP MY WAYS. If, however, you explain it as the Targum, “I know “of” him that he will command his sons etc.”, then the word למען does not fit into the sense.

3יצוה HE WILL COMMAND — This form of the verb (the imperfect) expresses frequentative action, as (Job 1.5) "This Job used to do" (יַעֲשֶׂה)

4למען הביא IN ORDER THAT [THE LORD] MAY BRING — Thus will he ever command his children saying “Keep the way of the Lord in order that the Lord may bring upon Abraham etc.” Since it does not say here ‘‘upon the house of Abraham”, but upon Abraham himself, we may infer that he who trains up a righteous son is as though he never dies (Genesis Rabbah 49:4).