Most verbs are regular enough that all forms can be derived directly from the infinitive. For example, the verb "love" derives all its forms systematically ("love, loved, loving, has loved, loves"), and since these can all be deduced from the basic form (the citation, dictionary, or lexicographic form, which in English is the bare infinitive), no other principal parts have to be learned. With irregular verbs like the verb "sing", on the other hand, the forms "sang" and "(have) sung" cannot be deduced, so the learner of English must memorize three principal parts, "sing–sang–(have) sung". The present 3S (he/she/it) is derived from the infinitive+'s' with the exception of verbs ending in a single -o, or ending in -s, -x, -z or the digraphs -sh, -ch, in which case it is derived from 'infinitive+es'. With three irregular verbs (and their derivatives) the 3S has to be learnt independently (e.g. has, does, undoes, redoes).