*in combustion reactions CO2 + H2O are always products. The rest depend on how many carbons in the fuel
*iron + oxygen --> rust (water is not a byproduct although it does play a part in the reaction)
*you can think of precipitation reactions like
AB + CD -> AD + BC
where A, C are positively charged and B and D are negatively charged. see if those two compounds are broken up (like if they are in water perhaps) then the positive ion will join with the negative of the other compound and the negative ion will join with the positive of the other compound, making two new compounds.