How do you conjugate regular -AR, -ER and -IR verbs in the pretérito indefinido?

Take the infinitive of the verb (the whole verb e.g. hablar) then remove the ending (AR, ER, IR) so you are left with the stem, which is ´habl´here. Then add the correct ending depending on the person it refers to. AR - é, aste, ó, amos, asteis, aronER/IR - í, iste, ió, imos, isteis, ieron