If we supply a great deal of rich & interesting comprehensible input, i+1 is automatically present.
Teaching in the Zone (August 2017).
Published in Language Magazine, October, 2017.
Sara Davila defines "i+1" as a task just above the acquirer's current level of understanding, introduced "to provide a challenge."
I defined "i+1" in terms of the natural order of acqiuisiton: i+1 is the next structure(s) the acquirer is ready to acquire.
In my original statements and in a more recent paper, I presented evidence for the hypothesis that we need not, and should not, aim at i+1. If we supply a great deal of rich and interesting comprehensible input, i+1 is automatically present. We don't have to "target just the right spot" for optimal acquisition. Please see Krashen, S. 2013 The Case for Non-Targeted, Comprehensible Input. Journal of Bilingual Education Research & Instruction 15(1): 102-110, available (Free download) at:http://www.sdkrashen.com/articles.php?cat=6 (scroll down).
For a more recent view of "targeting," and when it might be useful, please see: http://skrashen.blogspot.com/2016/11/three-options-non-targeted-input-and.html.