Isaiah 40:18

וְאֶל־מִי תְּדַמְּיוּן אֵל וּמַה־דְּמוּת תַּעַרְכוּ לוֹ׃

18 To whom will you compare God? And what image will you compare to him?

וְאֶל־מִי תְּדַמְּיוּן אֵל To whom will you compare God? The form תְּדַמְּיוּן is a piel imperfect, 2 m. pl from דָּמָה, "be like, resemble" with a paragogic nun (the nun is not necessary for the form, but makes better euphony with the following word). The piel intensifies the meaning toward "compare, consider to be like." The preposition אֶל expresses the direction "towards" God within the comparison. Notice the clipped form of God's title, אֵל el, often the title assigned to false gods, but here used as a challenge within the comparison: Is there really any el but the true el? Notice that the comparison is from the perspective of God: the munah accent under תְּדַמְּיוּן connects that word strongly to אֵל, almost as strongly as a maqqef would, were it possible in this construction: To whom will you compare God?

The Mp note draws our attention to Mm list 1560: the phrase וְאֶל־מִי occurs four times: 1 Sam. 6:20; Isa. 40:18; 40:25 and Job 5:1. Another note tells us that the form תְּדַמְּיוּן, with or without its paragogic nun, is a hapax legomenon.

וּמַה־דְּמוּת תַּעַרְכוּ לוֹ And what image will you compare to him? With דְּמוּת "image, likeness" the passage now turns as if on a hinge. I don't know whether we might grammatically call this verse a Janus passage (one that looks forward and backward), but it does in fact look both ways. The "likeness" in the previous verses has been God contrasted to the nations, now the "likeness" will be God contrasted with idols. The parallel term for "compare" is the qal imperfect תַּעַרְכוּ, which has an idea of "lining up" the way a child might arrange a row of dolls or toy soldiers. Who could anyone ever raise up, next to him? (לוֹ of comparison)