ועשו ארון עצי שטים… ועשית בדי עצי שטים (תרומה כה:י.יג)

Why is the first passuk in singular form and the second one plural? There are 613 mitzvahs – commandments – but not all of them can be performed by everyone.

For example, there are mitzvahs that apply solely to a Kohein, like Pidyon Haben, Terumah etc., which excludes the bulk of Klal Yisroel from ever being able to partake in them. 

To solve this problem, the Yidden must keep b’achdus – united, which is considered as if they performed these mitzvahs.  Therefore it says in plural format because every Yid can perform all mitzvahs when they are united together.