Your verb must agree with your subject in number and person. If your subject is singular, your verb must be singular; if your subject is plural, your verb must be plural. If your subject is in the first person, your verb must be in the first person. If your subject is in the second or third person, your verb must agree.
For more than a century, researchers has known that
exposure to high pressure can injure or kill living organisms.
For more than a century, researchers have known that
exposure to high pressure can injure or kill.
--"The Physiology of Decompression Illness," Scientific American
Do not be led astray by modifying phrases that separate the subject and the verb. If you are not sure about the subject-verb agreement, test the sentence by leaving out the modifiers.
A mixture of materials were used to
withstand high temperatures.
[The subject is mixture, not materials.]
A mixture of materials was used to
withstand high temperatures.
Also see subject-verb agreement involving two or more nouns, collective nouns, indefinite pronouns, quantifiers and sentences beginning with there.