The concept of a sentence is simple, if we only think of the written form of a language that has a punctuation system roughly similar to that of English. A sentence normally starts with an uppercase letter and ends with a period “.”, a question mark “?”, an exclamation mark “!”, or sometimes an ellipsis “…” indicating premature ending.
A sentence consists of one or more clauses. A clause contains a predicate verb, i.e. a verb in a finite form, such as “is” or “helps” or “wrote”, as opposite to forms like “to be” or “being”. The following sentences in English and in Finnish contain three clauses, with the predicates underlined:
He was thinking what he should do next, and then the phone rang.
Hän mietti, mitä tekisi seuraavaksi, ja sitten puhelin soi.
The example illustrates how Finnish uses commas between clauses more often than English does. The rules for using the comma are one of the key reasons why the clause concept is relevant in Finnish grammar.
Although Finnish often uses commas between clauses, a sentence cannot be divided into clauses simply by splitting at commas. Commas are also used also inside clauses, and commas are omitted between clauses in some contexts.
For adequate analysis of a sentence, we need to recognize the finite forms of verbs, i.e. predicates, and interpret other words as belonging to the same clause. Some words, like mitä and ja in the example above, will then be analyzed as constituting “glue” that joins clauses, such as relative pronouns and conjunctions.
Analyzing a clause is more difficult in Finnish than in English, because the word order varies more freely and because you will not find most words as such in dictionaries. A word typically has inflection, which needs to be recognized before you can look the basic form up in a dictionary. This is made more complicated by the fact that many suffixes have multiple uses. If you see a word like sanomme, you can recognize mme most probably as a suffix, but this same suffix is used both in some verb forms and as attached to nouns.
This means that recognizing predicate verbs requires both knowledge about verb inflection and dictionary information. The word form sanomme can be resolved as a verb form, because there is verb with the stem sano- but no such noun. Sometimes there are complications here. For example, the word uskomme can be analyzed both as a verb form (we believe) and as a noun with a possessive suffix (our faith). The context is needed to distinguish between them. If there is no other word in the sentence that looks like a verb, you would analyze that uskomme is probably a verb form.
Here is a complete sentence in Finnish, one that you might encounter when filling out a form:
Jos haluat, että olemme yhteydessä sinuun, kirjoita puhelinnumerosi tähän.
It is a long sentence, and you can hardly recognize more than one part of one word only (numero), if you did not learn any Finnish before reading this. However, you can tentatively divide it into parts by assuming that commas separate clauses. Here this gives a correct analysis of the overall structure. An analysis of the words:
Thus, a more or less literal translation would be “If you wish that we will be in contact with you, write your telephone number here.”