Drawing glass can be difficult, in this exercise two pencils and the lid have been added to show how objects distort though the jar. It is really important to consider value in all drawing but particular with glass, water and shiny objects. Value is the word for darks and lights. Getting glass to look right is all about analysing the value of the tones.
Another very tricky aspect of a glass or a jar is drawing ellipses...many of my students really struggle with them. They vary in shape according to how high up towards your eye level they are, narrowing to almost flat as they rise and opening to almost round the lower they are.
A good way to add tone is to then cover the whole drawing in the lightest possible shade of tone, then add incrementally more where it is slightly darker, and again until you get to the darkest areas. You can always rub away at the drawing to fine tune it. Your rubber is a mark making tool not just a corrector.
Examples from my classes...