[John P]

How exactly do you know how to stretch out bonds and atoms in what spot? A good example would be C3H6O3.
How do we know that it needs a double bond with the Oxygen and why do we add the O to the H? These are the only things I can't understand in this structure.

I am referring to this bond image: http://wakatipu.schoolzone.net.nz/Internet_Science/Chemistry/bonding.GIF

Thank you very much

[Vidya]

I hope you have a good understanding of lewis dot structures and how to predict the shapes from it

[Arkcon]

You have to begin by understanding how many bonds an atom will form.  For example, look at the first carbon on the left.  How many bonds does it have, and how many atoms are bonded to it?  Now look at the carbon you're talking about, it has just as many bonds, but one less atom, how else are you going to get away with that?