In English a verb normally agrees with the subject of the sentence, not with a following complement.
- The biggest timewaster is appointments. (NOT The biggest timewaster are appointments.) (subject – biggest timewaster; verb – is; complement – appointments)
Here the singular verb is agrees with the singular subject timewaster.
- The biggest problem in our country is unemployed youngsters. (NOT The biggest problem in our country are unemployed youngsters.)
Here the singular verb is agrees with the singular subject serious problem.
However, sometimes the verb is made to agree with the complement. This usually happens when the subject is a long way from the verb.
- The only watchable thing on television last weekend was the football matches. OR The only watchable thing on television last weekend were the football matches.
This may also happen when the subject is a relative what-clause.
- What I am interested in knowing is / are his personal reactions.
- What we need is / are a few bright youngsters.
When a singular subject is modified by a following plural expression, people sometimes use a plural verb. This is usually considered incorrect.
- Nobody except his close friends like him. (More correct: Nobody except his close friends likes him.)