In Estonian, the plural form of declined word, i.e. nouns, adjectives, numerals, is indicated by infix 'de/te' or 'i' between the word root and case ending. Plural nominative and partitive differ since only ending 'd', 'sid/id' or vowel is used.  

You may wonder when to use 'de' or 'te'. The first is attached to the root ending with vowel and another to the root ending with consonant. Now little bit about roots. As with general declension, not all cases are created equal, i.e. some are more important and used to form others. Namely, singular genitive serves as basis for plural nominative and singular partitive without ending 't' is used for plural genitive which in turn is used to form all other cases starting illative.  

Once again I would like to suggest using Eesti keele süntesaator to create various forms. By entering the words, you have to use Estonian õ, ä, ö, ü where present.* If translating and having trouble in guessing the basic form, use Eesti keele lemmatiseerija. Once again have all the umlauts lined up if you want any results.

*With Windows, under Control Panel you’ll find Regional Settings where you’ll be able to configure an Estonian keyboard layout.

Case designation

Nimetav/nominative

Omastav/genitive

Osastav/partitive

Sisseütlev/illative

Click, to isten to the plural forms