Sometimes you have to play from a loud sound to a soft sound, so you are diminishing the sound, or "Diminuendo" the sound, from loud to soft, or from forte to piano.
A measure normally has all the beats that you need to complete a full measure, in a 4/4 time you need 4 notes, 4 quarter notes. But sometimes you just need 1 note in that measure, always in the beginning of the song. This "special measure" or incomplete measure is called "an Anacrusis" or a "Pick Up" measure. And you only have as many counts as you need, even when the measure is "Incomplete", the notes that you a re missing usually goes at the end of the song, or the last measure.
A time signature has 4 quarter notes, or 2 half notes (Notice that 4 is double as much as 2), then if you get half of 4 now you have 8. (8 is double as much as 4) so you can have 4 quarter notes, or 8 eighth notes, so the eighth notes last less time because you have more notes in the same space of time. Now if you get a smaller note then the eighth notes, you have the sixteenth notes, and you need 16 notes in the same measure, a lot of notes. each note is 1/16th of a measure.
The same as the 16th notes, you have a symbol for the rests, a 16th rest. And it last the same as a 16th note, 1/16th of a count.
About the volumen of the sound, you can have a piano or a forte sound, a soft or loud sound, in a volumen of 1 to 10 "Mezzopiano" will be about 5, more or less in the middle, not too loud, not too soft. Mezzopiano means "Middle Soft".
Try this test to see how much you know so far. You need to know 10 symbols in a row with no mistakes.