markers are: The consonant -т of the present tense marker assimilates to the 3rd plural personal ending: -ҫҫĕ. The past tense allomorph -р- is used after vowels, while -т- is used after consonants. The simple past tense is used only for witnesses events, whereas retold events are expressed using the past participle suffix -н(ӑ) (see below). In addition to the iterative past, there is also an aspectual iterative suffix -кала- expressing repetitive action. There are also modal markers, which do not combine with tense markers and hence have sometimes been described as tenses of their own: The concessive suffix -ин is added after the personal endings, but in the 2nd singular and plural, a -с- suffix is added before them: кур-ӑ-сӑн(-ин) 'alright, see it'. If the particle -ччӗ is added, the meaning becomes optative. Potentiality is expressed with the suffix -(а)й 'be able to'. The negative is expressed by a suffix inserted before the tense and modal markers. It contains -м- and mostly has the form -м(а)-, but -мас- in the present and -мӑ- in the future. The imperative uses the proclitic particle ан instead (or, optionally, an enclitic мар in the 1st person). A change of valency to a passive-reflexive 'voice' may be effected by the addition of the suffixes -ӑл- and -ӑн-, but the process is not productive and the choice of suffix is not predictable. Still, if both occur with the same stem, -ӑл- is passive and -ӑн- is reflexive. A 'reciprocal voice' form is produced by the suffixes -ӑш and -ӑҫ. There are two causative suffixes - a non-productive -ат/ар/ӑт and a productive -(т)тар (the single consonant allomorph occurring after monosyllabic stems). There are, furthermore, various periphrastic constructions using the non-finite verb forms, mostly featuring predicative use of the participles (see below). Non-finite verb forms Some of the non-finite verb forms are: I. Attributive participles Present participle: -акан (вӗренекен 'studying' or 'being studied'); the negative form is the same as that of the past participle (see below); Past participle: -н(ӑ) (курнӑ 'which has seen' or 'which has been seen'); the final vowel disappears in the negative (курман) Future participle: -ас (каяс 'who will go') Present participle expressing a permanent characteristic: -ан (вӗҫен 'flying') Present participle expressing pretence: -анҫи, -иш Necessitative participle: -малла (пулмалла 'who must become'); the negative is formed by adding the enclitic мар Satisfaction participle: -малӑх (вуламалӑх 'which is enough to be read') Potentiality