# For who knows what is good for a person ... he passes through like a shadow?

"No one knows what is good for a person ... he passes through like a shadow."

# in life, during the few and meaningless days he passes through like a shadow

"during his futile, short life, which he passes through as quickly as a shadow passes by"

# Who can tell a person ... after he is gone?

"No one can tell a person ... after he dies."

# what will happen under the sun

"what will happen on the earth". See how you translated "under the sun" in Ecclesiastes 1:3.