what is the ending for a verb (present tense - infinitive) to change it to -ing. Like practice, practicar (to practice) to practicing?
1st conjugation verbs (ar) - ando
2nd & 3rd conjugation verbs (er, ir) - iendo
The thing that you attach these endings to are referred to as a verb stem.
ar - practicando
explains some of the difference in the use of these words in Spanish and English.
See lorenzo's answer which is what I would have said.