This Finnish language lesson, titled "Sanatyypit OSA 3 | vanha i-sana ja KPT | Suomen kieli Katjan kanssa," focuses on the intricacies of "i-words" in Finnish, specifically distinguishing between modern and older Finnish words ending in the vowel 'i'. The instructor, Katja, explains that while all i-words end in 'i', their declension patterns vary significantly based on their age and origin. The video builds upon a previous lesson about "new" or modern i-words and now delves into the three categories of "old" Finnish i-words: normal old i-words, "Li, Ni, Hi" words, and "Si" words. Katja begins by explaining the "normal old i-words," which are common Finnish words like *ovi* (door), *sormi* (finger), *Suomi* (Finland), *nimi* (name), and *järvi* (lake). The key teaching point here is that when these words are declined, the final 'i' is replaced by an 'e'. For example, *ovi* becomes *ovea* in the partitive case, *oven* in the genitive, and *ovet* in the plural. She emphasizes that this pattern is straightforward, but the challenge lies in knowing which words fall into this "old" category, requiring memorization. A notable cultural reference is the word *Suomi*, which, being a country, does not have a plural form, highlighting a grammatical nuance tied to real-world concepts. The lesson then progresses to i-words that exhibit KPT-gradation, a common feature in Finnish grammar where consonants change depending on the grammatical case. Katja explains that these are typically "nature words" (*luontosanoja*) like *joki* (river), *mäki* (hill), *lampi* (pond), *lahti* (bay), *lehti* (leaf/newspaper), and *siipi* (wing/section of a building). She clarifies that KPT-gradation in these words follows an "A-type" pattern, meaning the basic form is strong, and the gradation moves from strong to weak, except in certain cases like the partitive and the "S-mihin" (into what/where to) forms, where the strong form reappears. Katja provides examples of how these words change, such as *joki* becoming *jokea* (partitive), *joen* (genitive), and *joet* (plural). She also points out that words like *lehti* and *siipi* have multiple meanings, enriching the vocabulary lesson. Finally, Katja introduces the more specialized categories of "Li, Ni, Hi" words and "Si" words. The "Li, Ni, Hi" words, such as *kieli* (language/tongue), *suuri* (large), *pieni* (small), *sieni* (mushroom), and *lohi* (salmon), are also old Finnish words. While the 'i' still disappears and is replaced by 'e' in most cases, the partitive form is unique, ending in '-ta' or '-tä' (e.g., *kieli* becomes *kieltä*). She contrasts this with modern i-words like *peili* (mirror), which would have a standard partitive ending. The "Si" words, a smaller and more specific group including *uusi* (new), *vuosi* (year), *kuukausi* (month), *vesi* (water), and *käsi* (hand), have an even more distinct pattern. Here, the entire 'si' ending is removed, and the partitive takes a '-tta' or '-ttä' ending (e.g., *uusi* becomes *uutta*). The genitive and plural forms involve a 'd' (e.g., *uuden*, *uudet*), and the "S-mihin" form can exhibit KPT-gradation, making it a slightly more complex group. Katja concludes with a crucial call to action: learners must memorize which words