Gibbs, Jeffrey A. “Matthew 6:1-4: Alms." Matthew 1:1-11:1. St. Louis: Concordia Publishing House, 2006, pp. 309-313.
Gibbs notes that Matthew 6:1-7:6 "contains a remarkable alternation of eight present imperatives and twenty-four aorist stem imperatives" (Gibbs 2006, 309). The specific reason or interpretation indicated is not altogether clear, but Gibbs does observe the present imperatives here normally introduce new ideas, while the ideas are expanded upon by the aorist imperatives.
In Matthew 6:2, the identity of a hypocrite bears examination. Gibbs says, "The first-century denotation of the term 'hypocrite' does not necessarily include purposeful feigning of false motives. Someone can be a hypocrite without knowing it" (Gibbs 2006, 310-311).
This second part of the Sermon on the Mount shifts our attention from the application of Torah to the motivation of acts of piety (Gibbs 2006, 312). Gibbs observes that 6:1 introduces the entire unit, which is a triad, each following the same pattern. The pattern, broken when Jesus moves to the Lord's Prayer, makes that deviation all the more striking (Gibbs 2006, 313).