Answer:
Formal Language:
- This is a serious problem.
- I hope to hear from you.
- Thank you for your time.
Informal Language:
- This is totally ridiculous.
- No way that's happening!
- You're the best!
Explanation:
We use Formal language when we are talking to people we do not know well, in formal situations and in academic speeches or texts. It is characterized by the use of complex sentences, lack of colloquial terms and the use of polite and formal expressions.
On the other hand, we use Informal language when we are talking to people we already know (our friends and family) in everyday situations. It's characterized by the use of simple and colloquial sentences, slang and lack of formal expressions and structures. 
 
        
             
        
        
        
The invention of technology had made our life easier. Communication, work, and transportation had evolved over time. We no longer travel longer, we no longer has to work manually and there are so many gadgets and appliances to help improve our lifestyle.
Indeed, it had made our lives easier and better.
However, there were also downsides of its intrusion to human society. Children no longer play outside.  People have become lazy and would rather rely on technology for important things. People no longer see face to face, but rather on the screens of their smartphones, tablets, pads and computers.
Technology may have been one of the best things in life, but because of technology, we no longer appreciate the little things that life has to offer.
 
        
             
        
        
        
The type of error in the sentence above is case. 
The wrong case of the pronoun is used - instead of nominative case, it should be accusative, or object case.
<em /><em>Neither of us students was going to the library.</em>