'Do' and 'does' are the two forms from the present indefinite tense from the verb 'to complete.' Which one particular you will use will mainly rely on the sentence's topic. Their use is sort of identical, apart from 1 occasion where You need to use 'do' and not 'does.' https://financefeeds.com/valantic-fsa-appoints-ex-deutsche-borse-holger-wohlenberg-as-ceo/