Matthew 28:19  Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
20  Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.

The word translated "teach" is mathteuo, make disciples. It is in the aorist imperative tense, and conveys a sense of urgency, equivalent to Do It Now! The aorist also conveys the idea of "once for all", a sense of finality.

The word for "to observe" is tereo, which carries the implication of "keep an eye on", "guard", "obey", as the habit of their lives.