#No, but Sarah your wife will bear

God said this to correct Abraham's belief that Sarah could not have a son.

#you must name him

The word "you" refers to Abraham.

#As for Ishmael

The words "As for" show that God is switching from talking about the baby that would be born to talking about Ishmael.

#Behold

"Look" or "Listen" or "Pay attention to what I am about to tell you"

#will make him fruitful

This is an idiom that means "will cause him to have many children." (See: [[:en:ta:vol1:translate:figs_idiom]])

#and will multiply him abundantly

"and I will cause him to have many descendants"

#leaders of tribes

"chiefs" or "rulers." These leaders are not the twelve sons and grandsons of Jacob that will lead the twelve tribes of Israel.

#But my covenant I will establish with Isaac

God returns to talking about his covenant with Abraham and emphasizes that he would fulfill his promise with Isaac, not with Ishmael.