changing the font of a statement to Roman

while trying to write (if) in the Roman font, I came across this
\textnormal{} % to use the LaTeX font of the main body text (usually serif)
\textrm{} % to force Roman font (serif)
\textsf{} % to force sans serif font

but these commands do not work, does this library support other way for forcing the roman font on parts of the text ?