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You can use several methods like, charge on the ion, compound, or lewis structure (of electron sharing covalent bonds) to find the oxidation states of the elements in side a compound.

You can sum up oxidation states of atoms inside the compound to get the ionic charge or oxidation state of the compound.

oxidation state of Hydrogen is +1.  So carbon has -4 in compounds like CH4. But it has different oxidation states in different compounds.
CH3 has a charge +1 and it exists in gaseous form and is produced in ionic form when a proton reacts with methonol.  Its oxidation state will be +1.  The ion CH3 forms a compound  methyl iodide,  CH3 I.  Iodine has an oxidation state -1. Hence CH3 has an oxidation state +1.

COOH - has a structure :  O = C - O - H.
As oxygen has an oxidation state -2, and hydrogen has +1,  OH has -1.  The other oxygen and hydroxide combined have 3 bonds with C.  One electron is there for sharing with C.  Hence,  it has -1 oxidation state.  It forms compounds H- COOH (methonoic acid) and  CH3 - COOH (acetic acid). 

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