They were able to do this by substituting sounds. In all of the languages spoken by the slaves there was a common use of the letter D. Absent from their language was the use of several combinations of letters found in Standard English. One example is the combination of the "th" sound used for them, that, there, and other words, of course, you'll be able to name yourself.
Therefore, the substitution of sound occurred for these words -- think about it -- the "D" for the "th", making the African-American Folk language sound of dem, dat, and dere. When the "th" was at the end of the word, it was eliminated and the "f" sound used,giving the sound of mouf, teef, and so on.
Copyright 2000