Monikko <> The Plural
The basic plural form (monikon nominatiivi) of any word is easy to work out. It's simply the singular stem +t.
Kirja ---> Kirja- ---> Kirjat
Vene ---> Venee- ---> Veneet
Käsi ---> Käde- ---> Kädet
Tehdas ---> Tehtaa- ---> Tehtaat
Kasvis ---> Kasvikse- ---> Kasvikset
Nainen ---> Naise- ---> Naiset
Mies ---> Miehe- ---> Miehet
Lapsi ---> Lapse- ---> Lapset
Remember, when you use an adjective it has to be in the same form as the noun, so adjectives can also become plural in Finnish.
Kaunis nainen ----> Kauniit naiset
Hyvä kauppa ----> Hyvät kaupat
Valkoinen hattu ----> Valkoiset hatut
Väsynyt opiskelija ----> Väsyneet opiskelijat
Remember also that this is only the basic plural form, know in Finnish as monikon nominatiivi or monikon perusmuoto.
I suggest that to learn the different plural forms in this category you read each page carefully and then practice by just saying the words. The following 4 pages might be a bit confusing. Try not to worry too much about remembering the actual rules. A good way to learn is by picking one word of each type and just practice saying it in all the different plural forms over and over again:
For example: Practice saying the word kauppa in all of the following forms; the stem, the partitive, the genitive and the illative.
Kauppa, kaupoi-, kauppoja, kauppojen, kaupoihin
When you've done it enough, you won't forget it, and when you come across a similar word you'll know how to find the plural forms.
Kissa, kissoi-, kissoja, kissoihin
talo, taloi-, taloja, taloihin
In my opinion, this is the best way to learn these plural forms, because then you don't confuse yourself with all the complicated grammar (which there's enough of in this language already). Give it a try. First, read the 4 pages in this section, then just practice by speaking. I think you'll find it very effective.