Is a reduplicated verb necessary after "ba3"? In my textbook there's an exercise in which I must decide if a sentence is correct. There's this: 请把这些汉字练. In the answers they say it's incorrect because "lian4" should be reduplicated. Is it really a mistake? I always thought reduplicating a verb is optional, is a matter of what you really want to say (if you want to soften the sentence or not).
Quote from "Chinese an essential grammar":


  ......, a 把 construction must have an object of definite reference (shifted now to a pre-verbal position directly after 把); a complement of some kind after the verb to
  indicate the result achieved by the action of the verb, either intentionally
  or unintentionally, on the part of the subject. ......


Quote from "Chinese Grammar Wiki": Key Things to Keep in Mind When Using 把:


  把字句 are most often used to describe what happened to the object in some detail. The verb is not just "bare"; there's "more stuff" after it. Often the "stuff" is related to some kind of manipulation of the object.


The 把 structure just does not work with a bare verb. Reduplicating the verb also works, as it also adds some information (compliment) to the verb.