Chizuk Emunah Under the Microscope: Chapter 36

And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and pleas for mercy, so that, when they look on me, on him whom they have pierced, they shall mourn for him, as one mourns for an only child, and weep bitterly over him, as one weeps over a firstborn. (Zechariah 12:10)

Troki believed that this prophecy is about Israel in the last days, the ones where Israel finally triumphs over God and Magog. The other nations will look on the Jews and mourn for them. Specifically, Troki does not believe that this can apply to Jesus because they look upon one person and mourn for another. Therefore, the one that they pierced cannot be the subject of mourning.

The grammar in this verse is very awkward. The word for pierced is Strong's H1856, and occurs 11 times. Only once is it used in the 3rd common plural form (דָּקָ֑רוּ), which is this verse. One would expect a 1st person ending to be added onto the end, yet no such ending is on this word. The second problem is that the one they pierced should be the same subject as the subject of mourning, yet it is not.

The simplest solution to the problem is that the passage has a textual error. The vav at the end of (אֵלַ֖י) was lost. It was originally (אֵלַ֖יו) "upon him." Many of the manuscripts in the Masoretic library have this rendering of the verse, and would read as follows 

(  וְהִבִּיטוּ אֵלַיו אֵת אֲשֶׁר־דָּקָרוּ וְסָפְדוּ עָלָיו כְּמִסְפֵּד עַל־הַיָּחִיד)

They will look upon him who they pierced and they will mourn for him as one mourns for an only child.

This reading has been preserved in the Talmud (Succah 52a) and in the Masoretic library.

Just as importantly, the "upon me" rendering better supports a Christian translation. With the doctrine of the Trinity, they can look upon God and mourn for him, having both as the same being but different persons. They will look upon God the Father who they pierced by crucifying Jesus, and will mourn for him as one mourns for a child.