How can some people unerringly complete your sentence?

People are startlingly accurate in their ability to make these predictions, not only in terms of the basic content of the message, but also in terms of the word choices and phrasing of the sentences.

This ability to effectively predict the syntax of others in context comes from our knowledge of "linguistic probability".

An Australian-American team of investigators has made novel discoveries about the human ability to predict what other people are about to say.
"Linguistic patterns are important in predicting comprehension. If we can make better use of these patterns to enhance comprehension, then we can improve people's ability to understand one another," said Joan Bresnan and Marilyn Ford, who co-authored the study.   

Their findings could have significant applications for educators, speech therapists, entrepreneurs, and many others interested in communication and comprehension.
The greater the amount of experience that individuals have of a language, the greater their ability to predict. This is true of different dialects within a language, said a release of the Linguistic Society of America.

For example, Australian speakers of English and American speakers of English detect slightly different patterns of phrasing and usage among their respective fellow speakers, thus enabling them to more effectively predict the syntax that will be used in a variety of contexts.

This intrinsic ability to predict based on probability has implications for language comprehension.

Entrepreneurs engaged in marketing their products or services might use the most probable phrases in preparing their advertising messages.
These findings may also help in making computerised language more natural. Another practical application would be in the refinement of tools used in profiling and diagnosing those with language disorders.
The study is slated for publication in the March issue of Language.