# When Saul saw David

"When Saul had seen David" or "Earlier when Saul saw David". This (17:55-56) happened before David killed Goliath. 

# go out against the Philistine

"go to fight against the Philistine"

# whose son is this youth

"who is this youth's father"

# As you live

This was a way of swearing that what he was about to say was true.