Which aspects of the Tlingit verb are unpredictable?
The good news is, only the stem itself is unpredictable. All of the other prefixes are totally predictable. The stem is the last part of the verb word. For example, the stem in the word aawaxáa “s/he ate it” is –xaa. The stem in the word yak’éi “he/she/it is good” is –k’éi. Specifically, what is unpredictable about the stem of a Tlingit verb in a given tense/aspect (past, present, future, etc.) is the vowel length (long or short) and the tone (high or low). For example, given the form kashxeet “s/he is writing”, we cannot predict whether the verb stem will have a long and hi vowel: -xéet, a long and low vowel: -xeet, or a short and hi vowel: -xít in any other tense, such as the future or perfective. As it turns out, this verb stem has a short high vowel in the perfective: kawjixít “s/he wrote” and a long high vowel in the future: kakgwashxéet “s/he will write”. Compare this pattern with the verb al’eix “s/he is dancing”, which has a long, low vowel in the stem in all three forms; the imperfective (just given), the perfective aawal’eix “s/he danced”, and the future akgwal’eix “s/he will dance”. These patterns are not predictable, but must be learned for each individual verb.