Questions & Answers about Bardzo długo czytałam tę książkę.
How do we know the speaker of this sentence is female?
Look at the verb czytałam. In the Polish past tense, singular verbs change based on the gender of the speaker. The -łam ending is used for women. If a man were speaking, he would use the masculine ending and say czytałem.
Why is there no Polish word for 'I' in this sentence?
The word for 'I' (ja) is already built into the verb's ending. The -m at the very end of czytałam tells us that the first person (I) is doing the action. You only need to add ja if you want to emphasize it.
Why does 'this book' change to tę książkę instead of staying as ta książka?
Because the book is the direct object being read, it takes the accusative case. For feminine nouns and pronouns in this case, the -a endings change to -ę. That is why ta książka becomes tę książkę.
Why is the word długo used to mean 'for a long time'?
In Polish, you usually just use the adverb długo (which literally means 'long' or 'longly') to describe an action taking a long time. You do not need extra words to translate the English phrasing 'for a' or 'time'.