Tense is the form of the verb that shows distinctions in time. It expresses the time at, during, or over which a state or action occurs. Therefore, verb tenses must be formed correctly to give the clear meaning of a sentence.
A verb has three simple tenses:
The present tense expresses a present action or condition. It is formed by using the base form of the verb or by adding -s or -es to the base form if the subject is in the third person and is singular.
One of the hackers' favorite weapons against computer systems is the virus.
Some viruses crash a computer by creating many copies of themselves.
Time expressions like usually, now, today, always, and every day indicate present action.
The past tense tells us that an action has happened already.
It is formed by adding -d or -ed to the regular verb.
Irregular verbs form their past tense in a different way.
Robert Morris created one of the most notorious viruses.
The virus disabled thousands of computers.
Words like yesterday, last week, two months ago, last year, etc. indicate past actions.
The future tense tells us that the action is yet to happen. It is formed by using the word shall or will before the base form of the verb.
Users will unknowingly spread viruses.
Words like tomorrow, two days from now, next week, next year, etc. indicate future actions.