Here's why:
* The stem changes in certain conjugations: The stem "escrib-" changes to "escrib-" in the present tense, but in the past tense (preterite and imperfect), the stem changes to "escrib-".
Examples:
* Present tense:
* Yo escribo (I write)
* Tú escribes (You write)
* Él/Ella/Usted escribe (He/She/You write)
* Preterite tense:
* Yo escribí (I wrote)
* Tú escribiste (You wrote)
* Él/Ella/Usted escribió (He/She/You wrote)
* Imperfect tense:
* Yo escribía (I was writing)
* Tú escribías (You were writing)
* Él/Ella/Usted escribía (He/She/You were writing)
The stem change happens in the first person singular, second person singular, and third person singular forms of these tenses.