Verb

[Unregistered]

I want to enquire which one of the following sentence is correct and why?
A large number of organisms are seen.
A large number of organisms is seen.

[Casiopea]

I want to enquire which one of the following sentence is correct and why?
A large number of organisms are seen.
A large number of organisms is seen.

If you want to refer to the 'number', then use a singular verb:

A large number of organisms is seen.

If you want to refer to the 'organisms', then use a plural verb:

A large number of organisms are seen.

Please read below. I pulled the information from the University of Toronto's Writing page.

With fractions, percentages and indefinite quantifiers, the verb agrees with the preceding noun or clause. With singular or non-count nouns or clauses, use a singular verb:

One third of this article is taken up with statistical analysis.
All of the book seems relevant to this study.
Half of what he writes is undocumented.
About fifty percent of the job is routine.
All the information is current.

With plural nouns, use plural verbs:

One third of the students have graduate degrees.
Fifty percent of the computers have CD-ROM drives.
Many researchers depend on grants from industry.

With collective nouns, use either singular or plural, depending on whether you want to emphasize the single group or its individual members:

Half of my family lives/live in Canada.
All of the class is/are here.
Ten percent of the population is/are bilingual.

2. The words majority and minority are used in a variety of ways. When majority/minority mean an unspecified number more or less than 50%, use a singular verb:

The majority holds no strong views.
A small minority indicates it supports the proposal.

When majority/minority mean a specific percentage, you may use either singular or plural:

A 75% majority have/has voted against the measure.
A 10% minority are/is opposed to the measure.

When majority/minority refers to a specified set of persons, use a plural verb:

A majority of Canadians have voted for change.
A minority of the students wish to pay more.

3. Expressions of time, money and distance usually take singular verbs:

$10 is a great deal of money to a child.
Ten kilometres is too far to walk.
Six weeks is not long enough.

4. Adjectives preceded by the and used as plural nouns take plural verbs:

The rich get richer.
The poor have many troubles.

5. Expressions using the phrase number of take singular verbs when referring to a single quantity:

The number of students registered in the class is 20.

They take plural verbs when they are used as indefinite quantifiers (see rule 1 above):

A number of students were late.