The V infinitive has logically the suffix mainen : maise- : mais-, but it appears in the adessive plural only and always followed by a possessive suffix, so the total suffix is mAisillAni, mAisillAsi, mAisillAAn (∼ mAisillAnsA), mAisillAmme, or mAisillAnne.
The V infinitive is used only in conjunction with the verb olla (to be), and the expression means that someone is or was just about to do the thing expressed by the verb. It is mostly used in past tense forms and then indicates that the action was in fact not carried out. Thus, Olin ostamaisillani sen means that I almost bought it, i.e. was really going to buy, but some¤thing prevented it. The action need not be intentional. We can say e.g. Olin putoamaisillani katolta (I was about to fall down from the roof). This would be used to refer to a specific situation where falling down was really near.
The V infinitive, when associated with a past tense form of olla, describes a past situation in the context of describing past events. In contrast, special use of the I infinitive as in Olin pudota describes a past event from the perspective of the present: I almost fell down, i.e. I was near falling but didn’t fall. Since the V infinitive describes a situation, the text context normally contains a clause that says what happened in that situation, e.g. Olin ostamaisillani sen, kun vaimoni tuli ja kielsi minua (I was about to buy it, when my wife arrived and told me not to).
When used in present tense, the expression indicates a firm intent to do something but in a manner that suggests that it might still be prevented. For example, Olen ostamaisillani tämän talon means that I have serious intentions for buying this house, even so that I am about to buy it right now, and it may call for objections or comments. In contrast, a near-future construct like Olen ostamassa tämän talon means that I am going to buy this house soon, but probably not immediately.
All expressions described in this section are rare in speech. Instead, simpler verb forms and adverbs are used, e.g. Mä olin putoomassa katolta (using standard forms: Olin putoamassa katolta) = Olin putoamaisillani katolta, Mä melkein putosin katolta = Olin pudota katolta, and Mä meinaan just ostaa tän talon = Olen ostamaisillani tämän talon.