But if a man live many years, and rejoice in them all; yet let him remember the days of darkness; for they shall be many. All that cometh is vanity.
But if a man live many years, (and) rejoice in them all. So Vulgate, supplying 'and.' Rather, as Chaldaic, Syriac, and Arabic, translate, without ellipsis, 'For' х kiy (H3588)] or 'Yea, even if a man live many years, let him (not tire of life, but) rejoice in them all.'
Days of darkness. But while man thankfully enjoys life, "let him remember" it will not last forever. The 'many days of darkness' - i:e., while the body and its powers lie in the dark grave (Job 10:21-22); also days of "evil" in this world (Ecclesiastes 11:2) - are coming; therefore enjoy life, and saw the good seed while life and good days last, which are not too long for accomplishing life's duties.
All that cometh - i:e., All that followeth in the evil and dark days is vain, as far as work for God is concerned (Ecclesiastes 9:10).