This question already has an answer here:

What is the origin of the progressive form of the english verbs

marked as duplicate by RegDwigнt Sep 20 '14 at 21:35

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • @RegDwight The linked question addresses the origin of the -ing form, but says nothing about what is asked here. The origin of the progressive constructions in which that form is used is, I believe, a matter of some controversy; and the elaboration and use of those constructions is a process which is still in progress. – StoneyB Sep 20 '14 at 21:42
  • The Wikipedia article on it suggests it may be borrowed from Celtic. That would explain why German doesn't have a progressive construction -- no Celts to calque. – John Lawler Sep 20 '14 at 21:48
  • @JohnLawler That's one theory; against it lies the fact that the progressive seems to have arisen first in Late Old English, in the North: a time and place when the principal external influence on spoken English would have been Norse rather than British. Another theory is that it was originally a literary form which arose under the influence of Latin. At any rate, it took a long time to become grammaticalized: it was not common until the 16th century, its forms were not completed until the early part of the 19th; and it is still evolving in AAVE and Caribbean dialects. – StoneyB Sep 21 '14 at 0:06
  • This class handout has a deeper treatment and a bibliography. – StoneyB Sep 21 '14 at 0:09
  • Yeah, it definitely is still evolving and has many regional and social variants. Between around 1850 and 1925 or so in the US, sentences like The bridge is building gave way to The bridge is being built for the same meaning -- "Under Construction". The change regularized the separation of the patient subject bridge as due to Passive, and the active participle building as due to the Progressive. So one can now call it a Passive Progressive, in form as well as function. A lot of syntactic changes go in this regularization direction. Same as ain't, but that went too far. – John Lawler Sep 21 '14 at 0:18