In the first example, you could actually use either preposition: 1) Nowadays, mobile phones are important to us. Nowadays, mobile phones are important for us. They both are correct and they mean almost the same thing. "to us" will focus on that we personally value mobile phones. "for us" may mean that we recognize the importance of mobile phones in our lives (even if we personally don't value them). Hopefully that helps and I didn't confuse you more! This website may be helpful in learning prepositions: http://www.ego4u.com/en/cram-up/grammar/prepositions