A paragogic nun and a paragogic ה are sometimes suffixed to a word for reasons that no one knows. LaSor writes §13.73:
The addition of a sound at the end of a word might be called opisothesis, and the sound could be termed opisothetic. However, the term paragogic is commonly used in Heb. grammars.
Nûn paragogicum is found at the end of many verb forms otherwise ending in -û ——וּ or -î ִי (Ges. §47m). As Joüon correctly points out (J. §44e, f), the -n belongs to a primitive form and is attested by cognate languages. It does not add “energy” to the form.
Hê paragogicum is found in cohortative and some imperative forms, cf. Joüon §45a.