Communication is a process which involves a speaker and an addressee.As we communicate,we must be mindful of our tenses and the way the words in the sentence agree with each other.Adhering to strict concord rules in English is keen to ensuring that one constructs grammatically correct sentences.
SOME CONCORD RULES.
Usually people have problems with the use of words which already have plural forms but are not plural subjects.As such they tend to use plural verbs for them.It must however be noted that some words originally have the -s marker added to them.
Example - mathematics*.
Somebody might not construe the principle well enough and as such may use a plural verb for the word mathematics ,physics
Example- Mathematics are difficult.
The above is wrong because the -s marker added to mathematics does not make it plural but rather a singular subject.Therefore the correct construction should be mathematics is difficult.
Again any subject in a sentence which talks about distance,money or an amount is regarded as a SINGULAR SUBJECT.
For example- Five kilometres is/are a long distance from here.
In the above the correct form the verb is is not are. One should not be confused because of the -s added to the kilometre because the subject in the above sentence relates with distance and is therefore comsidered a singular subject.
Examples
1.Two thousand dollars is a huge amount of money.
2.Six O'clock is the time for the programme.