singular subjects need singular verbs and plural subjects need plural verbs

e.g.

The study aims to investigate the apparent correlation between these variables.

Results show that the project is feasible.

RULE: EVERY

"Every" is followed by a singular verb:

e.g. Every device is inspected before dispatch to customers.

RULE: HEADWORD OF THE NOUN PHRASE

The headword of the noun phrase determines whether the verb is singular or plural:

e.g. The handling of the earlier models was difficult during field work.

The sophisticated structure of the programs makes them hard to modify.

“There” may look like the subject of the sentence, but the real subject is the noun that follows the verb:

 e.g.  There were several limitations which affected the outcome.

There were two successful attempts to bring the device safely to the ground.

If the subject of a relative clause is a relative pronoun, pay special attention to which noun the relative pronoun refers to:

e.g. The most expensive material which was available had the highest durability rating.

These are the only samples which indicate some degree of contamination.