The simple rule is: "-a" is past, "-ura" is present, "-vEn" is future.
Note: the forms listed below are actually third-person forms but in our dialect, we use them for first-person. Ex. Ponen is the correct way to say "I went", but we also say Pona for "I went".
VERB
Root
Past "I" form
Present "I" form
Future "I" form
GO
Po-
Pona (I went)
Pora (I go)
Poven (I will go)
DO
Seyye-
Senja (I did)
Seyyira (I do)
Seyyiven (I will do...and so on)
SAY
Selle-
Senna
Sellura
Selluven
SHOW
Kaatta-
Kaattina
Kaattura
Kaattuven
WATCH
Paaka-
Paaththa
Paakura
Paakuven
SEE
Kaanu-
Kanda
Kaanura
Kaanuven
ASK
Keeka-
Ketta
Kekkura
Kekkuven
COME
Vaa-
Vandha
Vaara
Vaaruven
CLIMB (or GET IN)
Era-
Erina
Erura
Eruven
GET DOWN (from a car, etc)
Eranga-
Erangina
Erangura
Eranguven
SIT or EXIST or STAY
Irukka-
Irundha
Irikkira
Irikkiven
WALK
Nadukka-
Nadundha
Nadukkura
Nadukkuven
SLEEP
Paduththa
Padukkura
Padukkuven