Stem modification is a morphological process whereby an affix occurs simultaneously with a root or stem.
The kind of affix involved in this process is called a simulfix.
(English)
The tense/aspect morpheme in the English verb paradigm of 'to sing' occurs simultaneously with the verb stem:
This page is an extract from the LinguaLinks Library. Version 5.0 published on CD-ROM by SIL International, 2003.