One group of Finnish nouns that can cause problems for beginners is the nouns that end in –i in the nominative singular, but have a stem ending in –e– in all other forms, such as nimi, nimen ‘name’ and talvi, talven ‘winter’.
One solution, of course, is simply to memorize all of these words separately. This is actually a much less arduous task than one might imagine, since the nouns that fall into this class tend to be words for basic concepts that have been in the language for a long time without being replaced by borrowings, and hence there are only so many of them (and you will eventually get a feel for which sorts of words are likely to undergo this change). However, we can do better.
What I would suggest when confronted with these nouns is to learn the stem, not the nominative singular form as your dictionary may assume you will. Thus, you would memorize nime– and talve– rather than nimi and talvi, and then remember that the –e– becomes –i in the nominative singular (which is in fact what happened historically, although it is unclear what the quality of the original vowel at the end of these stems actually was). You will need to be careful not to confuse nouns with a stem ending in –e– and a nominative singular in –i with nouns that have a stem in –ee– and a nominative singular in –e, but if you always write nouns of the first group with a hyphen after the stem, that should help you keep them separate.
Another rule worth remembering is that a noun whose inflected forms show a stem in –e– (not –ee-!), and that isn’t a personal name, a very obvious loanword, or one of a small set of irregular words, usually falls into this group and will have a nominative singular ending in –i. Do remember to account for consonant gradation though, and also bear in mind that –te– at the end of a stem becomes –si in the nominative singular, when there are no endings after it (hence vesi, veden ‘water’, with the stem vete-). This rule can create homonyms in the nominative singular only: kuusi can mean either ‘six’ or ‘spruce’, but the two words have different stems (kuute– and kuuse– respectively).
This is a short post because there isn’t all that much to say here. If you still find it confusing, leave a note in the comments or feel free to ask directly!