A still more powerful word - perhaps the most powerful in the entire English language - is "not," which changes the very sign of a sentence from positive to negative, altering meaning by 180 degrees. The simple word "not" may not only change the meaning of a sentence but its political significance as well. Consider the implications of (1) "Life does begin at inception" versus, (2) "Life does not begin at inception."
Even changing a single letter of a single word can dramatically alter meaning. "He broke a law" means something very different from "He broke a jaw," or "He broke a claw."
Finally, even changing the location of empty spaces between letters can dramatically affect meaning. For a simple example, consider the difference between "I am used" and "I amused." Recently, I read a book (I think it was a work of fiction) containing several more interesting and complex examples of changing sentence meanings by moving spaces; unfortunately, I cannot recall which book.
UPDATE: The book I was trying to think of was Richard Russo's latest (and wonderful) novel, That Old Cape Magic (Knopf 2009), in which Russo describes a baffling sign above a bar, which reads:
heresto pands pen dIt looks like nonsense or some inscrutable Middle English quotation, until the spaces are reordered and punctuation marks are added:
asoci al hourin har
mles smirt hand funl
etfri ends hipre ign
bej usta ndkin dan
Devil spe akof no ne.
Here stop and spend a social hour in harmless mirth and fun. Let friendship reign. Be just and kind and evil speak of none.