Final answer:
The tú form for -er/-ir regular verbs in the preterite tense in Spanish is formed by removing the -er/-ir ending and adding -iste to the verb stem, such as 'comiste' for 'comer' and 'viviste' for 'vivir'.
Step-by-step explanation:
The tú form for -er/-ir regular verbs in the preterite tense is constructed by removing the -er or -ir ending and adding -iste to the stem of the verb. For example, for the verb 'comer' (to eat), you would take off the -er and add -iste to form 'comiste' (you ate). Similarly, for the verb 'vivir' (to live), it would become 'viviste' (you lived). This formula is consistent across all regular -er and -ir verbs in the preterite tense.