"Sf" means "sforzando". It is Italian for accented or forced.
sforzando
Sforzando in music means an accented chord
sf, or sfz, is short for 'sforzando' which means accented, or forced.
The symbol sfz or sometimes sf stands for sforzando. when the sign appears above a note, play that note or group of note loudly and then resume playing at the previous volume before
Anne Mischakoff has written: 'Sforzando!' 'Khandoshkin and the beginning of Russian string music' -- subject(s): Criticism and interpretation, History and criticism, Music
Sforzando - 2011 was released on: USA: 24 July 2011 (DVD premiere)
that is a musical marking for sforzando which means to suddenly become louder on the note or phrase.
sforzando, like an accent, suddenly playing the note loudly and accented
I am not sure what music theory you are looking for. Music theory is very vast and required time and practice to know it completely. You can check out some music theory lessons at pianomother and from there you can learn the basic up to the advance lessons for music theory.
You should look to find music theory lessons for your child by hiring a personal music teacher with a degree in music theory, or that have at least taken music theory classes.
Journal of Music Theory was created in 1957.