Verbs are often flexible with regard to valency. In non-valency marking languages such as English, a transitive verb can often drop its object and become intransitive; or an intransitive verb can take an object and become transitive. For example, in English the verb move has no grammatical object in he moves (though in this case, the subject itself may be an implied object, also expressible explicitly as in he moves himself); but in he moves the car, the subject and object are distinct and the verb has a different valency. Some verbs in English, however, have historically derived forms that show change of valency in some causative verbs, such as fall-fell-fallen:fell-felled-felled; rise-rose-risen:raise-raised-raised; cost-cost-cost:cost-costed-costed. In valency marking languages, valency change is shown by inflecting the verb in order to change the valency. In Kalaw Lagaw Ya of Australia, for example, verbs distinguish valency by argument agreement suffixes and TAM endings: Nui mangema "He arrived earlier today" (mangema today past singular subject active intransitive perfective) Palai mangemanu "They [dual] arrived earlier today" Thana mangemainu "They [plural] arrived earlier today" Verb structure: manga-i-[number]-TAM "arrive+active+singular/dual/plural+TAM" Nuidh wapi manganu "He took the fish [to that place] earlier today" (manganu today past singular object attainative transitive perfective) Nuidh wapi mangamanu "He took the two fish [to that place] earlier today" Nuidh wapi mangamainu "He took the [three or more] fish [to that place] earlier today" Verb structure: manga-Ø-[number]-TAM "arrive+attainative+singular/dual/plural+TAM" The verb stem manga- 'to take/come/arrive' at the destination takes the active suffix -i (> mangai-) in the intransitive form, and as a transitive verb the stem is not suffixed. The TAM ending -nu is the general today past attainative perfective, found with all numbers in the perfective except the singular active, where -ma is found. Tense, aspect, and modality Depending on the language, verbs may express grammatical tense, aspect, or modality. Tense Grammatical tense is the use of auxiliary verbs or inflections to convey whether the action or state is before, simultaneous with, or after some reference point. The reference point could be the time of utterance, in which case the verb expresses absolute tense, or it could be a past, present, or future time of reference previously established in the sentence, in which case the verb expresses relative tense. Aspect Aspect expresses how the action or state occurs through time. Important examples include: perfective aspect, in which the action is viewed in its entirety through