default verb form in Bangla?

i was looking at causatives in Bangla and observed that there is this general pattern of lowering of vowel in the verb stem when it is causativised. dekh-lo (see-pst), dAkh-a-lo (see-caus-pst) ; kin-lo (buy-pst), ken-a-lo (buy-caus-pst); khul-lo (open-pst), khol-a-lo (open-caus-pst). first i thought it was a case of vowel harmony with the causative marker being a low back vowel. but then looking at the hindi pattern i thought it might be something else.

hindi: ro-ya(cry-pst), rul-a-ya(cry-caus-pst); dekha-a(see-pst), dikh-a-ya(see-caus-pst).

since in hindi the default form of the verb is the noncausative form, then the causativisation causes vowel raising in hindi verb stems.

now my concern is:

if we can take the default form of the verb in bangla to be the stem in causativised verbs then, then bangla also seems to have a case of vowel raising in the verb stem, when the transitivity of the stem is changed. (not unaccusative-transitive transformation)

in fact, since the 2ndP-sg form or infinitival form of the verb is dAkh-, ken-, khol-, which are identical to the causativised verb stem, then may be we can take it to be the default form of the verb.

the broader question to pursue, if this turns out to be correct, will be to find that whether a number of IA lgs have this general trend of vowel raising in the verb stem when transitivity is changed.

ishani.

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