Questions & Answers about Когда я был дома, я читал книгу.
Why do we use дома here instead of в доме?
The word дома is a special adverb that simply means "at home". If you used в доме, it would emphasize being physically "inside the house" rather than the general state of being at home.
How would this sentence change if a woman were speaking?
Russian past tense verbs must match the gender of the speaker. A female speaker would say Когда я была дома, я читала книгу. Notice how both past tense verbs, был and читал, add an -а at the end to show the feminine form.
Why use the verb читал instead of прочитал?
читал is an imperfective verb, which is used for continuous or unfinished actions ("was reading"). If you used the perfective verb прочитал, it would mean the action was completely finished ("read the whole book").
Why does the word книга change to книгу?
The word книга is the direct object here (the thing receiving the action of reading), so it must be in the accusative case. For feminine nouns ending in -а, this means changing the ending to -у.