Finnish nouns ending in –e in the Nominative are a slightly odd group. Normally, when a Finnish word ends in a vowel, you simply add the endings directly after the vowel, possibly changing the stem slightly in the process. However, this group is different.
If we take a second to delve into Finnish historical linguistics, we learn that all of the native Finnish words ending in –e in the Nominative in the modern language (which is quite a few words) used to end in a consonant, which got worn down to a glottal stop over time (which is not written but is still pronounced in the Nominative!). In cases other than the Nominative, consonant gradation has eliminated this consonant entirely, but not before speakers had already inserted another epenthetic –e– before the endings. Hence, these words now have a stem ending in –ee in all forms other than the Nominative Singular.
The other interesting feature of these nouns is that their consonant gradation pattern is reversed compared to what you might already be used to on other nouns: the Nominative Singular gets the weak grade, and all other forms get the strong. However, this is unsurprising if we think about it for a moment or two — the Nominative Singular used to end in a consonant, and thus a closed syllable, triggering the weak grade, whereas in the other forms this consonant had slid over into the following syllable (to go with the epenthetic vowel), leaving the original last syllable of the stem open. Thus, we get words like sade, sateen ‘rain’.
It is worth bearing in mind that some words, especially names and very recent foreign loans, don’t actually belong to this group, and are declined as if they ended in a different vowel. However, these words are usually easy to identify, and you probably won’t confuse anyone too much if you get one or two declensions wrong.
This has been a short post, but it’s something worth knowing!