how do i do soft and loud, diminendo and cresendo? i found doing dynamics on the cello very difficult

Bowing faster, using more bow pressure on the string and playing nearer the bridge make the sound louder; slower, less pressure, nearer the fingerboard for quieter. However these differences also change the sound quality, so you usually need to change more than one of them at a time, and some combinations produce effects that are hardly ever wanted. For instance, if you bow fast with insufficient pressure eventually the bow will skid over the string and make a wispy noise; conversely, if you try to bow too slowly with lots of weight, you may produce a grungy noise, sometimes known as a sub-harmonic, or the bow may stop altogether.

Bow faster - Louder
Bow slower - quieter

Apply pressure and decrease pressure will also affect it, although you will have to practice to get it right. A good exercise to practice with is to select a string and just go up and down with the bow wit crecs. and dims. and then goin from quiet at the heel to loud in the middle and back down. It will also help with being able to hold notes longer.

Hope this made sense.

Good luck.