Aspect in the past: process or result

In the past tense Russian makes you choose between the two verbs of an aspect pair.

• Use the imperfective (читать → читал) for a process, a duration or a repeated action. Typical signals: весь вечер, долго, два часа, часто, всегда, раньше, в детстве.
• Use the perfective (прочитать → прочитал) for one completed action with a result. Typical signals: за два дня, наконец, как только, уже when the result is what counts, or a chain of actions one after another.

Compare: Я весь вечер читал книгу. (I was reading a book all evening.) and Я прочитал книгу за два дня. (I read the book in two days.)

The past tense agrees with the subject: он читал, она читала, мы / они читали. With я and ты both the -л and the -ла form are correct.