El pretérito grave

El pretérito grave is an irregular form of the preterite tense but with a pattern nonetheless. In fact, the only difficulty is having to memorise the irregular stem which is different from the infinitive stem. You just add the following endings to the stem:

e imos

iste isteis

o ieron

You may have noticed that the endings are identical to the ER and IR preterite endings except the 1st person singular and the 3rd person singular forms are like the AR preterite endings but without the accents.

Example with Poner

Puse Pusimos

Pusiste Pusisteis

Puso Pusieron

So, instead of the stem being Pon as usual, you have to remember it is Pus and then simply add the above endings which are easy to remember once you are proficient in the regular preterite endings.