What to do when my introductory sentence is taken from another author's paper?
I think some of the early advice on this question was really bad. It's easy to give casual attribution for a quote if you do it like this:


  Abstract : This paper is devoted to the study of K-algebras. It has
  long been known that 'All regular K-algebras are equivalent, but every
  irregular K-algebra is irregular in its own way.' (Tolstoy). We
  classify a new family of irregular K-algebras. This result has
  implications in algebra theory.


Then you can cite the paper properly in the body of your paper. Writing the abstract like this makes it very clear who the quote came from and you can deal with the where later.