There are 3 rules that changes the verb root in the preterite form:
- The -car, -gar, -zar rule
- Triple vowel rule
- Pancake rule
The -car, -gar, -zar Rule (YO FORM ONLY)
So this rule is pretty straight forward. If a verb ends in -car, -gar, or -zar (in the infinitive) then you drop the -car, -gar, or zar. After you drop it, you add -gué, -qué, or -cé (REMEMBER THIS IS ONLY IN THE YO FORM).
Example of this:
llegar -->llegué(yo form)
Triple Vowel Rule (-ER AND -IR VERBS ONLY)
The Triple Vowel Rule only refers to the él/ella/Ud and the ellos/ellas/Uds. forms. Some verbs have a double vowel in the last 3 letters of the word. Examples: leer & caer. When you have a double vowel(-er and -ir verb), conjugating(él/ella/Ud and ellos/ellas/Uds.) in the preterite becomes more complicated. Normally leer would conjugate to leió in the él form. This creates a triple vowel. To counteract this, exchange the i for a y. leió --> leyó. Another example: Caer --> cayó.
The Pancake Rule (-IR VERBS IN THE ÉL, ELLA, UD. AND ELLOS, ELLAS, UDS. FORMS ONLY)
This rule is more complicated and requires the verb to fulfill 2 requirements. The verb needs to:
- MUST Be a Boot verb IN THE PRESENT FORM
- Be a -ir verb
If only one vowel is changed (In the present form), you keep that vowel in the preterite. Example: in the pedir conjugation, the él/ella/ud & ellos/ellas/uds both keep the vowel "i" (its underlined in red).
If two vowels are changed (In the present form), you keep the first vowel in the preterite. Example: in the dormir conjugation, the él/ella/ud & ellos/ellas/uds both drop the second vowel, and keep the first one(underlined in red).