According to linguists, there are good reasons to think that in the proto-Uralic language from which Finnish has evolved, the word stems of “content words” (nouns, adjectives, numerals, and verbs) had two syllables, whereas pronouns had one syllable. This structure has largely been preserved in Finnish, though many common content words now have one syllable only, e.g. tie (road), due to phonetic changes. There are also many three-syllable words that may look like base words but are probably derivations of two-syllable words that have been lost, e.g. pihlaja. Most pronouns have now two syllables or even more; they have been formed by joining two one-syllable words or a word and a suffix. This explains the somewhat odd inflection of many pronouns, e.g. joku : jonkun : jotakuta, which historically consists of jo and ku, which are both inflected.
The two-syllable base word structure still affects the way Finns analyze words. A four-syllable loanword such as arkkitehti (architect) or papukaija (parrot) is often treated as if it were a compound word. This is reflected in inflection, e.g. arkkitehdeillä (with the vowel ä selected according to the tehti part) instead of the regular arkkitehdeilla. Splitting such words into two-syllable constituents may have other implications, too; e.g., the part kaija has been treated as a separate word when creating Finnish names for parrot species.
Made-up new words very often consist of two syllables, such as hetu, abbreviated from henkilötunnus (personal identity number; literally: person identifier).
When a word has more than two syllables, it can mostly be decomposed into smaller meaningful parts, either as a compound word or as a derived word, mostly with a two-syllable word as the ultimate base. For example, sanakirja (dictionary) is a compound of sana (word) and kirja (book), and johtaja (director) is derived from the verb stem johta- (to direct, to lead) with the suffix ja. However, there are exceptions: some base words have more than two syllables for various reasons. In any case, long words are not as difficult as they may look like. The word rautatieasema becomes understandable when you know its constituent words: rauta (iron), tie (road, way), and asema (station)—so it means railway station. In principle, the word asema is decomposable into the verb base ase and derivative suffix ma, but this is not particularly useful for learning the language, since the verb ase- (to be, to stay) has as such vanished from the language; it only lives in its derivations.