1. Yo no sé SI Jake compró un libro ayer (o no). = I don't know WHETHER Jake bought a book yesterday (or not). 2. Yo no sé SI Mary tiene un perro. = I don't know WHETHER Mary has a dog. 3. ¿Sabes SI Mark fue a la playa ayer? = Do you know WHETHER Mark went to the beach yesterday? 4. He didn't say WHETHER she had went to the store. (Sorry, I don't know how to say this one in Spanish.) 5. It doesn't matter WHETHER you're poor. (I don't know how to say this one in Spanish either.)

http://dictionary.cambridge.org/dictionary/british/whether?q=whether...or http://dictionary.cambridge.org/grammar/british-grammar/if-or-whether These pages will help you to understand the uses of "whether".