Answer:
2. Ella _está_ en el baño. Reason, current location (estar)
3. ¿De dónde _sóis_ vosotros? ¿Madrid? Reason, origin (ser). For example, yo soy de Costa Rica.
4. Yo _soy_ inteligente. Reason, a characteristic of the subject (ser), example: yo soy alto, delgado, inteligente, tonto.
5. Ella _está_ en Hawaii para una boda. Reason, current location (estar).
6. La ceremonia _es_ a las siete y media. Reason, event set in time (ser). Examples: Mañana es mi cumpleaños, hoy es domingo, mañana es mi reunión.
7. Nosotros _estamos_ tranquilos. Reason, current mood or state (estar), examples, estamos bien, estoy triste, ella está feliz. In this one tho, there's also the chance of being a characteristic of the people and not as a current mood. For example, I could be talking about my neighborhood, and I could say: Es muy bonito y pacífico por aquí, nosotros somos muy tranquilos (Is very nice and peaceful here, we are very chill people) for example.
8. ¿_Es_ usted de los Estados Unidos? Reason, origin (ser).
9. Nosotros _somos_ amigos de Mercedes. Reason, relationship between people (ser), examples: Yo soy amigo de Juan, tú eres novio de María, Juan es compañero de colegio de Carmen.
10. ¿_Estás_ (tú) nervioso hoy? Reason, current state or mood (estar).
11. Tú _eres_ muy talentoso. Reason, characteristic of the subject (ser).
Explanation:
With Spanish we have to pay attention to the different situations to be able to identify if it is Ser o Estar. As a very superficial observation, Ser tend to be more permanent (soy estadounidense, soy una persona tímida o extrovertida) and Estar tend to be temporary (estoy en mi habitación, estoy triste, estoy pensando).
About where my knowledge is from, I am not a pro-Spanish professor haha, but I am a native Spanish speaker, and as a uni student, I have to write a lot with a not-so-bad Spanish haha, and I have helped many of my friends who are not-native-speakers to learn and improve their Spanish, so I really hope this explanation is useful to you.
Good luck!
P.S Sorry for my not-so-good English haha.
