Good compositions should swell and subside, should punctuate moments of extreme emotion and melodic peaks with louder dynamics:
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You can signify dynamic changes in the sheet music with Italian words that signify basic descriptions of loud and soft. "Piano" means that you should play softly, and is usually written below the staff when the music should be played quietly. "Forte" means loud, and is written in the same way. Note the original name of the Piano, the Piano forté; this may help you in remembering that one of the exceptional features of the instrument is it's ability to be a percussion instrument (that also utilizes strings) that can both increase and diminish in sound. If you're not intending a great amount of dynamic contrast in your piece, or don't want to worry about this yet, or you prefer to focus on tonality and rhythm while learning to write, you may consider it's older relatives, the pipe organ and the harpsichord, which have different strengths and will help your fluency on piano.
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Gradations can be suggested by drawing an elongated "<" or ">" sign under the staff, where the music should either crescendo (get louder) or diminish your sound, depending.