Here the prophet is told the meaning of the metaphor and simile in the previous verse. Humans do not live long just like grass and flowers.
The grass withers, the flower fades: These are general statements about the nature of grass and flowers. Good News Translation expresses them more naturally in English with “Grass withers and flowers fade” (similarly New Jewish Publication Society’s Tanakh). Even though these statements have in view the frailty of humans, they may also refer indirectly to the imminent downfall of Babylonia.
When the breath of the LORD blows upon it: When renders the Hebrew particle ki. Like Revised Standard Version, many versions understand it as introducing a time clause here. However, it may be a logical connector here, introducing the reason why plants wither. New International Version and King James Version view it this way by translating it “because” (see the first example below). Translators may render it either way. The Hebrew word for breath (ruach) can also mean “spirit” (King James Version) or “wind” (Good News Translation; similarly Revised English Bible). This term probably has a double meaning referring to a hot desert wind and to God’s spirit who sends it. If possible, translators should use a term that suggests this double meaning. Most English versions do this with the word “breath.” The word “spirit” is another possibility, but it should not imply the Holy Spirit (see the comments on 11.2).
Surely the people is grass is synonymous in meaning with “All flesh is grass” in the previous verse (see the comments there). The adverb surely makes it a stronger statement here. It may be rendered “truly” or “certainly.” For the whole line Bible en français courant has “It’s very true, people have the fragility of grass.”
For the translation of this verse consider the following examples:
• Grass withers, flowers fade,
because Yahweh’s spirit [or, breath] blows upon them.
For sure, people are [like] grass.
• Grass withers and flowers wilt
when a hot wind sent by the LORD blows over them.
Truly, people are nothing but grass.
Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .