Many a time have they afflicted me from my youth, may Israel now say:
From my youth — From the time that I was a people.
 The LORD is righteous: he hath cut asunder the cords of the wicked.
Righteous — Faithful or merciful.
The cords — Wherewith the plow was drawn. By these cords he understands all their plots and endeavours.
 Let them be as the grass upon the housetops, which withereth afore it groweth up:
House-tops — Which there were flat.
Which — Having no deep root, never comes to maturity. And so all their designs shall be abortive.
 Neither do they which go by say, The blessing of the LORD be upon you: we bless you in the name of the LORD.
The blessing — Which was an usual salutation given by passengers to reapers: so the meaning is, it never continues 'till the harvest comes.