Hello there,*

I am trying to learn to draw and have started doing still life last week. Now, I understand that I have to do loads, not just 2 pieces, long and short, and fail in order to learn. However I feel I do not understand what I have to look out for, what kind of errors I need to see.

These are my studies:
Name:  stilllife (7).jpg
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Name:  stilllife (1).jpg
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Name:  stilllife (6).jpg
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Name:  stilllife (3).jpg
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Name:  stilllife (5).jpg
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Name:  stilllife (4).jpg
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Name:  stilllife (2).jpg
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The first four are still life drawings of a polystyrene head.
Length varies from 10 minutes to 1,5 hours.
Pencil only; 2B, 4B, 8B usually. A3 format.

I was actually surprised to find I can draw a hammer and apples and it would look like a hammer and apples. So for me, it's as far as it goes. What would a teacher see, though? I don't understand the purpose of still life. I can cite JeffX saying you can learn value and form and composition, but I am actually not aware of how that translates to a general drawing skill. I am especially confused about the use of medium. Shall I paint as well? In color?

I would be really grateful for any comments, tips or anecdotes you might have to make me understand what I have to learn. I have 2 jobs and not as much time as I would like to, so I would like to spend it as wisely as possible.
Thank you.

*I honestly didn't know where to open this thread. Art discussion was too general, CC/WiP to specific. So I gave fine arts a shot. Sorry for any inconvenience!