ואתה תצוה את בני ישראל (Tetzaveh 72:02) "And you shall command Bnei Yisrael." The first word of the pasuk, ואתה, is unnecessary because the second word, תצוה, starts with a "ת," which means "you".
A possible explanation is that God is teaching us how to deal with others. ואתה, "And you" - the way you act, the way you talk, the way you walk, will be the measure of your success in your תצוה, in your dealing with others.
If you want others to follow you, watch yourself first. If you want to impress good deeds and good characteristics onto others, practice what you preach.
Character cannot be taught; it must be caught. You must be caught up in a wave of influences and you must want to emulate the character of parents, rabbis, and great leaders, because you must become impressed by them.
A Rebbe must portray his own personality around his students if he wants them to follow in his footsteps. It is the rebbe's personality that becomes part of his students. They are caught up in the greatness of their rebbe.
Yehoshua was the most important student of משה רבנו. He became the leader of בני ישראל when Moshe died. Yehoshua served Moshe and was always around him. He observed him all the time. This molded Yehoshua into the great leader he became - ויהושע בן נון נער לא ימש מתוך האוהל.
The greatest obligation that a person has to his students and children is to inspire them to want to become a person of character and goodness.
We may not always be successful in the teaching the intellectual content of Torah. However, even when we aren't, we can be successful in teaching our children to grow up properly, to follow the laws and spirit of our holy Torah.