For they shall soon be cut down like the grass - As the grass in the field is cut down by the mower; that is, however prosperous they may seem to be now, they are like the grass in the meadow which is so green and luxuriant, but which is soon to fall under the scythe of the mower. Their prosperity is only temporary, for they will soon pass away. The idea in the word rendered "soon" - מהרה mehêrâh - is that of "haste" or "speed:" Psalm 147:15; Numbers 16:46; Deuteronomy 11:17. The thought is not that it will be done immediately, but that "when" it occurs it will be a quick and rapid operation - as the grass falls rapidly before the mower.
And wither as the green herb - When it is cut down. That is, not as the dry and stinted shrub that grows in the desert of sand, but like the herb that grows in a garden, or in a marsh, or by the river, that is full of juices, and that needs abundant water to sustain it - like the flag or rush (compare Job 8:11) - and that withers almost instantly when it is cut down. The rapidity with which things "wilt" is in proportion to the rapidity of their growth, so the prosperity of a sinner is suddenly blasted, and he passes away. Compare Psalm 90:5-6.

For they shall soon be cut down - They have their portion in this life; and their enjoyment of it cannot be long, for their breath is but a vapor that speedily vanishes away. They fall before death, as the greensward does before the scythe of the mower.

For they shall soon be (b) cut down like the grass, and wither as the green herb.
(b) For God's judgment cuts down their state in a moment.

For they shall soon be cut down like the grass, and wither as the green herb. Which in the morning looks green, pretty, and flourishing, and in the evening is cut down, and then fades away; see Psalm 90:5; and so the wicked prosper and flourish for a while, and then they perish with all their honour, riches, and wealth; so that their happiness is a very short lived one, and therefore need not be envied and fretted at.