"My sister is sitting and reading a magazine."
Translation:Моя сестра сидит и читает журнал.
every verb must be conjugated (there are of course instances where some would remain in their indefinite form, like after "надо,нужно...", when forming future tenses with imperfective verbs and so on). however, in general sentences like these, all the verbs should be conjugated when referring to the subject.
correct, magazine is in the accusative! if a masculine word is an inanimate object its accusative form will be the same as in nominative! (это журнал (nom.); я вижу журнал (acc.)). however, if a masculine word is describing a living thing (a person, occupation, animal etc.) its form in accusative will be the same as in the genitive case (это Иван (nom.) - я вижу Ивана (acc., notice how the suffix is the same as it is in genitive case! (gen.: у Ивана есть кот)
when it comes to recognising which case you should use, unfortunately, there are no simple tricks. when learning a language there are some things you just have to learn by heart. for example: some verbs, prepositions and phrases demand to be accompanied by specific cases... which again you are bound to learn by heart if you really want to learn the language.
some verbs, prepositions that are usually accompanied by an accusative case: • в (when asking where (куда)- мы едем в Москву, also when talking about what sport you play - я играю в футбол; в понедельник, в 5 часов) • на (мы едем на почту; сын похож на отца ("the son looks like the father"); он потратил деньги на книги;) • через (через час) • за (платить за покупку; спасибо за помощь) • видеть, любить, знать, читать, курить...