6. Expression necessity and obligation

Must is often used to indicate 'personal' obligation  what you think yourself or other people/things must do. The obligation comes from outside (a rule or law). 
Have to or need to are often used to indicate neccessity (but not always).   It means doesn't come from a rule or law.
Here are the expressions to show necessity and obligation :

 - It is necessary for you to be here on time.

 - You must finish your reports before the boss ask you.

 - We don't need to come to the party, I think.
 - You have to submit the report by June 2 ^{nd 
 - We must be here before the teacher comes}

 - The school must provide more sports facilities. 

 - You have to clean the keyboard regularly.
   - I really must get some exercise.   
   - People must try to be more tolerant of each other.
   - If you own a car, you have to pay an annual road tax.

5. Agreement and disagreement

The expression to show agreement we can use :

   - Exactly.

   - I agree with .......

   - Certainly.

   - I think so.

   - I totally/ extremely agree with you.

   - That's just what I was thinking.

   - Yes, that's quite tree.

Whereas to show disagreement we use expression as :

   - No way / non sense.

   - I'm afraid, I have my own opinion

   - I don't think so

   - That's not right

   - I'm sorry but you are wrong

   - Surely not, I can't agree with that ..........

   - I don't agree with you.