Грамматика русского языка. Часть 8. Глагол. Глава 9. Прошедшее время глагола, вторая группа (Продолжение). Russian Grammar. Part 8. The Verb. Unit 9. The Past Tense of The Verbs, The Second Group (Continued).

Some еть verbs (consonant stem) also have the Irregular Past Tense formation – no л in the Masculine Past, for example: умереть - to die.

So, the past Tense forms are as following:
он умер - he died
она умерла – she died
они умерли – they died

Грамматика русского языка. Часть 8. Глагол. Глава 8. Прошедшее время глагола, первая группа. Russian Grammar. Part 8. The Verb. Unit 8. The Past Tense of The Verbs, The First Group.

The Past Tense formation is based on the Gender Principal (Singulars).
The stem which serves as a base for the Past Tense formation comes from the Infinitive.
The Infinitives end in: ть, сть, чь, ти, сти, зть, зти.
As for the Past Tense formation all the verbs fall into two groups: