יערף כמטר לקחי. זוֹ הִיא הָעֵדוּת שֶׁתָּעִידוּ — שֶׁאֲנִי אוֹמֵר בִּפְנֵיכֶם תּוֹרָה שֶׁנָּתַתִּי לְיִשְׂרָאֵל שֶׁהִיא חַיִּים לָעוֹלָם כְּמָטָר זֶה שֶׁהוּא חַיִּים לָעוֹלָם (עי' ספרי); כַּאֲשֶׁר יַעַרְפוּ הַשָּׁמַיִם טַל וּמָטָר: יערף כמטר לקחי MY DOCTRINE DROPPETH AS RAIN — This is the evidence that ye will have to give: that in your presence I declare that the Torah which I gave to Israel (לקחי) is life to the world as the rain which is life to the world (cf. Sifrei Devarim 306:17) — even as the heavens drop down dew and rain.
יערף. לְשׁוֹן יַטִּיף, וְכֵן (תהילים ס"ה) "יִרְעֲפוּן דָּשֶׁן", (דברים ל"ג) "יַעַרְפוּ טָל": יערף means, IT DROPS. Similar is (Psalms 65:12) “[And thy paths] drop fatness” (ירעפון, where רעף = ערף); (Deuteronomy 33:28) “[also his heavens] shall drop down (יערפו) dew”.
תזל כטל. שֶׁהַכֹּל שְׂמֵחִים בּוֹ, לְפִי שֶׁהַמָּטָר יֵשׁ בּוֹ עֲצָבִים לַבְּרִיּוֹת, כְּגוֹן הוֹלְכֵי דְרָכִים וּמִי שֶׁהָיָה בוֹרוֹ מָלֵא יַיִן (ספרי): תזל כטל IT FLOWS AS THE DEW in which everybody rejoices. He adds this because rain involves annoyance to some people, as, for instance, to those on a journey, or a man whose pit (the pit into which the wine flows when the grapes are being pressed) is full of wine (which thus becomes spoiled) (Sifrei Devarim 306:17).
כשעירם. לְשׁוֹן רוּחַ סְעָרָה, כְּתַרְגּוּמוֹ "כְּרוּחֵי מִטְרָא", מָה הָרוּחוֹת הַלָּלוּ מַחֲזִיקִים אֶת הָעֲשָׂבִים וּמְגַדְּלִין אוֹתָם, אַף דִּבְרֵי תוֹרָה מְגַדְּלִין אֶת לוֹמְדֵיהֶן: כשעירים — This word has the same meaning as in (Psalms 148:8) “stormy (סערה) wind” (the ס and ש interchanging), as the Targum has it: כרוחי מטרא “like winds that bring rain”. How is it with the winds? They strengthen the herbage and promote their growth! So, too, the words of the Torah promote the moral growth of those who study them (Sifrei Devarim 306:18).
וכרביבים. טִפֵּי מָטָר. נִרְאֶה לִי עַל שֵׁם שֶׁיּוֹרֶה כְּחֵץ נִקְרָא רְבִיב, כְּמָה דְאַתְּ אוֹמֵר "רֹבֶה קַשָּׁת" (בראשית כ"א): וכרביבים — these are DROPS OF RAIN. It seems to me that it (the rain) is called רביב because it shoots down like an arrow, just as you say, (i.e. just as the text reads) (Genesis 21:20) “A shooter (רבה) with the bow”.
דשא. ארברי"ץ, עֲטִיפַת הָאָרֶץ מְכֻסָּה בְיֶרֶק: דשא — herbaries in old French. English herbage — the vesture of the ground when it is covered with vegetation (greens).
עשב. קֶלַח אֶחָד קָרוּי עֵשֶׂב, וְכָל מִין וָמִין לְעַצְמוֹ קָרוּי עֵשֶׂב: עשב HERB — a single stalk is called עשב and similarly each species of herbs by itself is called עשב (whilst דשא denotes vegetation in general). (Cf. Rashi on Genesis 1:11 and Note thereon).