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yoku = often, much
e.g Ryoko San wa yoku toshokan e ikimasu.
Miss Ryoko often goes to the library.
itsumo = always. Itsumo indicates that a behaviour or condition is routine, itsumo also indicates frequent occurence. Thirdly, Itsumo also expresses a certain established or regulated action or state.
e.g Kanojo wa, itsumo kono mise de kaimono o suru
She always shops at this store