Finnish hyphenation rules are relatively simple, and if a word processor or other program is reported to support automatic hyphenation for Finnish, it probably hyphenates most words correctly. However, there are three major problems:
For example, Finnish hyphenation in Microsoft Word sometimes divides a word between vowels or divides a compound word incorrectly. In contrast, some programs try to avoid wrong hyphenation of compound words too eagerly. Thus, some manual tuning may be needed. For high-quality publications, the final draft should be proofread to detect wrong hyphenations.
To hyphenate a Finnish word manually, first consider whether it is a compound word. If it is, regard the position between the parts as the best hyphenation point. For example, the word laivamatka (boat trip) is a compound, and laiva·matka is the best division. Here we use the middle dot “·” to indicate a permitted word division point.
This analysis requires knowledge about the meanings of words and therefore sometimes fails in any automatic hyphenation. For example, autonostaja (car buyer, a person buying a car) is to be analyzed as auton·ostaja, not as auto·nostaja (although the latter would be an imaginable compound, too, “car-lifter”). Automatic hyphenation might treat the word as au·tonos·ta·ja, avoiding incorrect hyphenation, but failing to use the best hyphenation point, if it cannot decide where the border between the parts is. This is what the online Webvoikko hyphenator does. In contrast, the online hyphenator at ushuai.pl produces au-to-nos-ta-ja, an incorrect hyphenation.
Next, find the syllable breaks in the word or in any component of a compound word as follows: a break appears before the last consonant in cluster of consonants (one or more) in any position except the start of a word (or component). Thus, laiva is divided as lai·va, matka as mat·ka, myrkky (poison) as myrk·ky and myrsky (storm) as myrs·ky. However, it is never permitted to separate just one letter of a word to a different line, so e.g. the word asia has no permitted hyphenation points, even though it has three syllables, a.si.a.
These rules do not give all syllable breaks, as defined in conventional grammars, but they give all acceptable hyphenation points. For example, the word radiossa (in radio) has the syllable structure ra.di.os.sa, but a division between vowels (except in a compound at component boundary) is regarded as more or less unsuitable, so the proper hyphenation analysis is just ra·dios·sa.
Hyphenation is not based on the division of a word into a stem and suffixes. For example, radiossa consists of the stem radio and the suffix ssa, but this is ignored in hyphenation.
Hyphenation between vowels is generally forbidden in Finnish typography, unless the vowels belong to different parts of a compound, as in iso.isä. There are two reasons to this rule. First, consecutive vowels often belong to the same syllable, at least in some pronunciations. It would therefore be inadequate to hyphenate e.g. vapa.udessa. Second, even when there is a syllable break, such division tends to make reading somewhat less smooth.
In official language rules, the rule is not absolute. It is presented as a recommendation that should be followed as far as possible. The reasonable interpretation is that the rule shall be followed, except in situations where breaking it helps to avoid a more serious typographic problem. The exception mainly applies to special situations in very narrow (e.g. newspaper) columns, where breaking e.g. asi.oita may help to avoid excessive spacing between letters for making the column justified on both sides.
In Finnish publications, it is however very common to violate the rule without such reasons. Automatic hyphenation does not always follow the rule and manual corrections are often regarded as too expensive and time-consuming.
Loanwords have some exceptions in hyphenation. Especially in older style and according to old recommendations, they may be divided according to the structure of the word in the original language. For example, abstrakti might be divided as abs-trakti (since “abs” is a prefix in the original Latin word), and demokratia (democracy) might be divided as demo-kratia (since the Greek original is a compound word).
Some dictionaries of international words (sivistyssanakirjat) contain information about hyphenation. It is, however, increasingly common to hyphenate according to normal Finnish rules, e.g. abst-rakti and demok-ratia.
If the second part of a compound word is a loanword that starts with two or more consonants, the simple hyphenation algorithm fails. For example, the simple algorithm analyzes veroprosentti as ve-rop-ro-sent-ti instead of the correct ve-ro-pro-sent-ti.