Here's some hint from web that maybe useful for you
How to make verbs agree with their subjects
1. What are verbs and subjects?
Verbs are action words -
e.g. eat, sleep, talk, walk, do, buy are all verbs
Subjects are the person or thing who are doing the action of the verb -
e.g.I eat; The dog sleeps; George talks a lot; They walk to work.
The subject of a sentence can be singular (one) or plural (many).
e.g. The computer is old. (singular)
The computers are old. (plural)
2. What is verb-subject agreement?
The verb form can change depending on whether the subject is singular or plural.
e.g. The car park (singular subject) was (verb) full.
The car parks (plural subject) were (verb) full.
In these sentences each of the verbs agrees with its subject. The correct verb form has been used.
The verb must always agree with its subject. Single subject = single verb, plural subject = plural verb.
3. How does this work?
In regular verbs:
singular plural
First person I like bananas. We like bananas.
Second person you like bananas. you like bananas.
Third person he / she / it likes bananas. they like bananas.
She likes to cycle to work.(correct) / She like to cycle to work.(incorrect)
We like swimming. (correct) / We likes swimming. (incorrect)
Helpful hint: 's' is added to the third person singular. This is the way most regular verbs in the present tense work.
Problems with verb-subject agreement
Irregular verbs
Not all verbs work in a regular way (see factsheet 1). Some of the most common verbs are irregular.
e.g. be, go, do, have
Verbs and subjects must still agree, but you have to learn and remember the way the irregular ones work.
Note the correct verb form for the third person singular for these irregular verbs:
'to do' = I do - he / she / it does (NOT do)
'to have' = I have - he / she / it has (NOT have)
'to go' = I go - he / she / it goes (NOT go)
e.g. She does karate on Thursdays. They do lots of sparring.
It has soft fur. They have soft hands.
She goes sailing every month. I go every week.
Too many words
Sometimes it's difficult to work out the subject, because there are lots of words between the subject and the verb
e.g.Steve, who has just returned from Australia, does not intend to go back.
(Singular subject, 'Steve' = singular verb)
The children, excited about Father Christmas, went to bed early without complaining.
(Plural subject, 'children' = plural verb)
The best thing on television last night was EastEnders.
(Singular subject, 'thing' = singular verb)
Problems with plurals in verb-subject agreement
Sometimes the subject (the person or thing doing the action of the verb) may seem to be plural, because it is a 'collective noun' - a singular (one) noun that groups together many things or people.
e.g. A swarm of bees = 1 swarm, containing many bees
A pack of cards = 1 pack, containing many cards
This is an area of some debate, but as they are treated as a singular unit, collective nouns usually take the singular verb form.
e.g. A herd of elephants was charging towards us.
The class is very noisy today.
My football team is doing really well.
e.g. My colleague and manager were both promoted today.
(Two people = plural subject)
Rupert and Jane are football fans but Colin prefers shopping.
(Two people = plural subject, takes the plural verb 'are'; one person = singular subject, takes the singular verb 'prefers')
e.g. The bag of shopping was too heavy to carry.
(Bag of shopping = singular subject. Lots of shopping, but there's only one bag.)
Remember: if you are unsure which verb form to use, look at the subject carefully. Is the subject singular (one), or plural (many)? A singular subject requires the singular verb form. A plural subject requires the plural verb form.
Regards,
Rina