Я села на лавочку у озера и открыла книгу.

I sat down on a bench by the lake and opened a book.

Breakdown of Я села на лавочку у озера и открыла книгу.

• я: I
• книга: the book
• открыть: to open
• и: and

Questions & Answers about Я села на лавочку у озера и открыла книгу.

Why is it села and not сел?

Because the past tense in Russian agrees with the speaker’s gender.

• села = I sat down said by a female
• сел = I sat down said by a male

The same applies to открыла:

• открыла = said by a female
• открыл = said by a male

So this sentence is spoken by a woman or girl.

Why is it села, not сидела?

These verbs mean different things.

• села comes from сесть and means sat down
• сидела comes from сидеть and means was sitting / sat

So:

• Я села на лавочку = I sat down on the bench
• Я сидела на лавочке = I was sitting on the bench

In your sentence, the speaker is describing a sequence of completed actions: first she sat down, then she opened a book.

Why is it открыла, not открывала?