Or HaChaim on Exodus 25:10/אור החיים על שמות כ״ה:י׳

1 ועשו ארזן, "They shall construct an ark." It is strange that G'd phrased the directive to build the ark in indirect speech whereas the directives to construct the Tabernacle, the Altar, the Table, and the Lampstand are all phrased in direct speech, i.e. ועשית. Not only does the Torah employ indirect speech in this verse, but when elaborating on the details of the construction the Torah reverts to use of direct speech, i.e. "you shall cover it with pure gold, etc." Perhaps we can find in this anomaly a hint that the bulk of Torah cannot be fulfilled by any individual but requires participation of the entire nation. No single individual can fulfil all the commandments of the Torah, however much he may desire to do so. If an individual happens to be a priest he cannot fulfil the commandment of allocating the 24 gifts which the Torah directs the Israelite to give to the priest. He cannot fulfil the commandment of redeeming his firstborn son. A Levite also is prevented from fulfilling certain commandments by the mere fact that he is a Levite. The phrasing of ועשו ארון in the indirect plural may be intended to remind us of this fact.

2Possibly, the use of the indirect plural here is an introduction to the construction of all the parts of the Tabernacle and its furnishings. By using the indirect plural at this point the Torah may wish to remind us that even where it uses the direct singular when giving directives, these directives are addressed to individuals only inasmuch as those addressed represent all the חכמי לב, (compare 36,1).

3It is also possible that seeing that prior to receiving the second set of Tablets Moses himself constructed a wooden ark (compare Rashi on Deut. 10,1) in which he placed them as soon as he descended from Mount Sinai, the instructions given here refer to an alternate ark, the one to be constructed by Betzalel. In order to distinguish between these two arks the Torah changed its grammar. G'd had already used direct speech when He commanded Moses to construct the original ark in Deut. 10,1. At that time He added the word לך,"for yourself," to indicate that pending the construction of the permanent housing of the Tablets, Moses was to construct a temporary home for them. Whereas the directives ועשית in our portion always refer to the construction by Betzalel of something permanent, the same word in Deut 10,1 did not refer to something permanent. Moses complied personally with G'd's directive to construct a temporary ark (Deut. 10,3) as we know from: "he placed the Tablets inside" (Deut. 10,5). Phrasing the present directive in indirect plural, i.e. ועשו, was to make plain that G'd spoke of an additional ark. Had G'd merely said ועשית, I would have thought that just as the original ark was meant to be his, personally, i.e. to be paid for out of his own pocket, so the permanent ark was also meant to be paid for by himself. Alternatively, Moses could have thought that G'd spoke of the very ark Moses had already made and that now it was to be overlaid with gold inside and out, have staves attached to it, etc. In order to prevent any misuderstandings, the Torah wrote ועשו.