The Finnish orthography is morphemic, and the morphemic notation is built upon the phonetic principle: with just a few subtle exceptions, within a single morpheme, each phoneme (distinct sound) of the language is represented by exactly one grapheme (independent letter), and each grapheme represents exactly one phoneme, if the morpheme is pronounced in isolation. This makes the language easy for its speakers to spell, and facilitates learning to read and write.
Some orthographical notes:
~ Long vowels and consonants are represented by double occurrences
of the relevant graphemes. This causes no confusion, and permits these
sounds to be written without having to nearly double the size of the
alphabet to accommodate separate graphemes for long sounds.
~ The n in nk is a velar nasal, like in English. As an exception to the phonetic principle, there is no g in ng, which is a long velar nasal as in English singalong.
~ The grapheme h occurring before a consonant sounds slightly harder (initially breathy voiced, then voiceless) than when occurring before a vowel.
~ Sandhi is not transcribed; the spelling of morphemes is immutable, e.g. tulen+pa /tulempa/.
~ Some consonants (v, j, d) and all consonants occurring in (always medial) clusters do not have distinctive length, and consequently, their allophonic variation is not indicated in spelling, e.g. raajaan /raajaan/ vs. raijaan /raijjaan/, or katko /katko/ vs. metsä /mettsä/.
~ Pre-1900's texts and personal names use w for v. Both correspond to the same phoneme, the labiodental approximant /?/, a v without the fricative ("hissing") quality of the English v.
The letters ä [æ] and ö [ø], although drawn as umlauted a and o, are nevertheless considered independent graphemes. An appropriate parallel from the Latin alphabet are the characters C and G (uppercase), which historically have a closer kinship than many other characters (G is a derivation of C) but are considered distinct letters.
How the Finnish letters ä and ö differ from their Germanic (German, Swedish) counterparts:
~ The Finnish sounds ä and ö, and their long counterparts
ää and öö, are grammatically independent, often
distinguishing unrelated words, e.g. talli "stables" vs. tälli
"punch". German umlauts often correlate with distinctions
of tense, mood, or plurality such as Rad/Räder for "wheel/wheels".
~ The pronunciation of ä and ö in Finnish does not change in diphtongs, or when followed by r, as it sometimes does in Swedish and German. The letters ä and e are not phonetically equivalent in Finnish.
~ The letter ä is very common, both by its own merits and by phonotactical reasons. Vowel harmony requires it for several grammatical endings such as the partitive case -ta/-tä, and it is also found in its long form, sometimes multiple times in a single word while contrasting with other forms, e.g. pää-äänenkannattaja "chief organ", tällä päivämäärällä "on this date"
~ In German, umlauts are replaceable: ä may be written ae and ö as oe. This is not possible in Finnish, as ae and oe are vowel combinations of their own right, with very different pronunciations. Minimal pairs exist between ä/ö and ae/oe, e.g. hän "s/he" vs. haen "I seek". The custom of replacing umlauts with oe or ae can produce silly and unpronounceable results, for example in TV broadcasts of sporting events, when applied to Finnish names like Eduard Hämäläinen -> "Eduard Haemaelaeinen". The preferred method, if äs or ös are not available, is to use simple a or o as in Kimi Räikkönen -> "Kimi Raikkonen".
~ Like in Swedish, the Finnish letters ä and ö are alphabetized as independent characters added to the end of the alphabet; the Finnish alphabet ends with "X Y Z Å Ä Ö". In German, the umlauted vowels are alphabetized together with their mother-characters, which is convenient given their grammatical role in German.
Finnish does not use the visually similar diaeresis notation, as used in French and English words such as coördinate or naïve. In such situations either hyphen (when the vowels belong in different syllables) or double vowel (when the question is about long vowel) is used: koordinaatti, naiivi.
For technical reasons or convenience, the graphemes sh and zh are often used in quickly or less carefully written texts instead of š and . This is a deviation from the phonetic principle, and as such is liable to cause confusion. In practice however, these letters are used nowhere else than in transcriptions (e. g. šakki, Tšekki, Saakašvili), so the damage is minimal. Finnish does not use the sounds z, š or , but for the sake of exactitude, they can be included in spelling. (The recommendation cites the Russian play Hovanshtshina as an example.) Many speakers pronounce all of them s, or distinguish only between s and š, because Finnish has no voiced sibilants.