Introduction:

           One of the most common types of grammatical error in Indonesia students’ English is called ‘agreement’. Agreement means that the verb changes depending on the type of noun it refers to. See below for examples. The grammar rules for agreement are reasonably clear and simple, and therefore, because these mistakes are so common in Indonesia students’ English, you can show that your English is superior if you follow the rules on this page.

Subject/verb agreement

          Subjects and verbs are definitely the most important participants in a sentence. Remember, the subject is the actor or acting concept in a sentence, and the verb does all the work and tells us what the subject is doing. They are always together. Therefore, it is very important that these two agree.


Examples :

•     ‘I like him.’ ‘I’ is singular and a first person pronoun, so the verb is ‘like’.

•     ‘She likes him.’ ‘She’ is singular, but a third person pronoun (so are ‘it’ and ‘he’), so the verb is ‘likes’.
•     ‘This student likes grammar.’ ‘Student’ is a singular noun, so the verb is ‘likes’.
•     ‘The dog likes him.’ ‘The dog’ is a thing, and is singular, so the verb is ‘likes’.
•     ‘Dogs like him.’ ‘Dogs’ is a plural noun, so the verb is ‘like’.
•     ‘The food here tastes nice’. ‘Food’ is uncountable, so use a verb that matches third person singular.
•     ‘Beer is cheap.’ ‘Beer is an uncountable noun in this sentence, so use ‘is’.
•     ‘These two beers are nice, but that one is horrible’. This means ‘These two types of beer’, so because there are two types, and types of beer are countable, use ‘are’. Drinks are often uncountable, but countable when they are types, in bottles or glasses; e.g. “Can I have an orange juice?”
• The expression the number is followed by a singular verb while the expression a number is followed by a plural verb.
  Examples:
  The number of people we need to hire is thirteen.
  A number of people have written in about this subject. 
• When a singular subject is connected by or or nor to a plural subject, put the plural subject last and use a plural verb.
  Example:
  The serving bowl or the plates go on that shelf.
   
• When a compound subject contains both a singular and a plural noun or pronoun joined by or or nor, the verb should agree with the part of the subject that is nearer the verb. The athlete or his teammates sprint every day.
  His teammates or the athlete sprints every day.
  

Notes :

• After words such as ‘must’ and ‘will’ use an infinitive verb without ‘to’; e.g. “He must tell me what happens.”

• The main verb should agree with the main noun, e.g. in “The research in these reports shows that I am correct.”, the verb shows agrees with research, as research is the main noun in the noun phrase The research in these reports. Research is an uncountable noun, so shows has an 's' at the end.

• Tense is more important than the number of subjects; e.g. The software will arrive tomorrow. Even though software is uncountable, and therefore should be followed by a verb with 's' at the end, tomorrow is more important, so the tense of the verb (will arrive) shows the tense.

Courtesy  :
http://www.myenglishteacher.net/subject_verb_agreement.html 
http://wiki.answers.com/Q/Examples_of_the_subject-verb_agreement
http://www2.elc.polyu.edu.hk/cill/exercises/sva.htm
http://www.csu.edu.au/division/studserv/my-studies/ess/subject_verb