"Teach, teaching - not correcting."
Maria Montessori said this many times in lectures, with a mischievious pause after saying 'Teach, teaching....' Then she would clarify for her confused audience: 'not correcting'.
In practice, this means that when you see a very young child making a mistake, say misspelling a word in a story, do not interrupt their work to correct them.
Instead, make a mental note of what the child did incorrectly, and later make sure to teach them what they have missed. For older children, correcting after the work is completed is a good idea; in this example having them study and take a spelling test immediately afterwards with the misspelled words is effective. Younger children seem more sensitive to being corrected directly, and a 2 to 4 year old may be discouraged from trying a new skill altogether if told they are 'wrong'.
More generally, this principle implies that one should use positive phrases and reinforcement when possible. "write neatly please" not "no sloppy writing"; "quiet voices" not "stop yelling". Putting directions in a positive voice is actually clearer to young children (so that the thing they are not supposed to do is not brought to mind) .
The best resources for more about Maria Montessori's methods are her books, including 'Montessori's Own Handbook' with detailed descriptions of materials and methods.
