(now archaic or literary) certainly, yes (indeed), but of course (used to contradict a negative assertion)

if, in case

if possible

if need be, if necessary

unless

youth is wasted on the young, you can't put an old head on young shoulders (it's too bad that young people lack the experience and wisdom which only come with age; likewise, it's a pity that old people lack the physical wherewithal to take full advantage of their experience and wisdom)

B-flat major

plural of langue de si

any of the Old Spanish dialects during the Middle Ages / any of the southern European Romance languages during the Middle Ages in which the word for yes was si, particularly Italian dialects, by opposition to langue d'oc and langue d'oïl

if need be