stupati

1. to step; to march
   Vojnici su ponosno stupali ulicom. — The soldiers marched proudly down the street.
2. (+ u + acc.) to enter; to enter into
   Mladi danas sve kasnije stupaju u brak. — Young people today are getting married later and later.
   Kompanije često stupaju u kontakt s nama. — Companies often get in touch with us.
3. (stupati na snagu) to come into force; to take effect
   Novi zakon stupa na snagu od januara. — The new law comes into force in January.

For a single completed action the perfective stupiti is usually used.