In older Japanese the Te Form and the plain past Ta Form (you can make the latter, just by changing -e to -a) used to be made by adding -te and -ta to the stem of the verb (what is left when you remove -masu).

This still works with all the Ru Verbs and U Verbs with Dictionary Form ending in -su:

For U Verbs, however, with Dictionary Forms ending in other letters, various contractions have taken place.

U Verbs ending in the nasal consonants bu, mu and nu end up with -nde:

Note that 死ぬ Shinu 'die' is the only verb in modern Japanese ending in -nu. Often it is replaced by euphemisms such as 亡くなる naku naru lit. 'become nothing, disappear' in polite speech.

With verbs ending in -u, -ru or -tsu, we end up with -tte:

With verbs ending in -ku or gu, the k or g in the stem drops, but the g has enough strength to turn -te to -de:

All verbs with the consonants mentioned follow these rules, as we would expect である de aru, する suru and 来る kuru are irregular:

行く Iku 'go' is also irregular in this one form: itte

言う Iu 'say' (pronounced yuu) is also irregular; also: itte