Finnish normally uses present tense even for future events and actions, e.g. Tulen huomenna (I will come tomorrow; literally: I come tomorrow). Typically, either the context or some adverbials expressing time make it clear that the future is referred to.
It is possible to use a compound future tense, formed using an auxiliary verb, the verb tulla “to come”. The main verb is in the mAAn infinitive form, e.g. tulen kirjoittamaan “I will write”, which literally means “I will come to write”. In many situations, such expressions are used in the concrete meaning (someone physically arrives somewhere). Usually no ambiguity arises, but sometimes both interpretations are possible, and there might be no strict line between the concrete meaning and the grammatical future. The future tense is mainly literary, so in speech, a sentence like Tulen auttamaan sinua normally has a concrete meaning (I will come and help you), instead of just promising to help in the future.
Language authorities generally recommend that constructs like tulen kirjoittamaan be avoided and present tense be used instead, if the context makes it clear that future actions are referred to. Many people take a stronger position and regard tulen kirjoittamaan as simply wrong or claim that there is no future tense in Finnish.
An older construct for expressing future is of the type olen kirjoittava (I will write), using the present participle. It appears in older language and has a solemn tone; the construct was used in Bible translations until 1992.
Such combinations of words may appear so that the participle is a predicative, e.g. olet hämmästyttävä. This was in principle ambiguous, with the meanings “you will amaze” and “you are amazing”, but only the latter is relevant in modern language. The form hämmästyttävä has more or less become an adjective, like English “amazing”.
When a construct like olen tekemässä is used with an object in the partitive case, it indicates ongoing activity, much like English “I am doing”, e.g. olen kirjoittamassa kirjettä (I am writing a letter). Thus, the meaning clearly relates to the present, but the action is expected to continue at least for some time. The form tekemässä is the inessive of the III infinitive and can be explained as meaning “in the process of doing”.
When the object is in another case, namely in a case for a total object, the expression refers to what is expected to happen in the near future, e.g. olen saamassa kirjeen häneltä (I am about to get a letter from him/her). The near future could here be later the same say or the next day, or even a few days ahead. In contrast, the olen ...mAisillAni construct relates to something that is about to be done immediately, though especially in spoken language, the olen ...mAssA construct is used even then.
In the absence of an object, an olen ...mAssA expression can be interpreted in two ways, though often the context and content make only one interpretation feasible. For example, joukkue on putoamassa kolmanneksi could be interpreted as “the team is about to fall down to the third place” (i.e. is expected to fall soon) or “the team is falling down to the third place” as something that is actually happening right now. An expression like olin nukkumassa can only mean “I was sleeping”, since sleeping is continuous action. In contrast, olin nukahtamassa is most naturally interpreted as “I was just about to fall asleep”, since falling asleep is normally understood as a momentaneous event.