Kinyarwanda

1. kumva => kumvira: ku-/-umv-ir-a => kutumvira: ku-ta-/-umv-ir-a => ukutumvira: u-ku-ta-/-umv-ir-a (rules: u becomes ø before a vowel; u becomes ø before a vowel).
2. gutwara: ku-/-twar-a => kudatwara: ku-ta-/-twar-a = ukudatwara: u-ku-ta-/-twar-a (Here Dahl's Law on Bantu language is applied: a voiceless stop, such as /p t k/, became voiced (/b d ɡ/) when immediately followed by a syllable with another voiceless stop.).
ku-twa-ra: ku-> gu because of t in twa

Also, look at negative ta which becomes da and ku- which replaces gu- before *-da-*.

ku-ta-twa-ra: ta->da because of twa

My problem: applying this law starts with the syllable near the stem? I have not yet got it.

Other examples:

*-t(a)-* before stem starting with vowel.

ta before stem starting with b (kutabana), d (kutadudubiza), g (kutagaburira), j (kutajarajara), m (kutaminina), n (kutanyara), r (kutarya), s (kutasama), t (kudatura), v (kutavura), w (I have not found a verb whose stem starts with w), y (kutayobora), z (kutazuka).

da before stem starting with c (kudacucuma), f (kudafuma), h (kudahaga), k (kudakaraba), p (kudapima),

Note: The stem starts with /-