Verbs are the crucial words in a sentence that indicate action or a state of being.
John hits the ball. (Hits expresses action.) Maria is a soccer player. (Is expresses being.)
Verbs have forms such as the infinitive and tenses that indicate when an action occurred. (Tense comes from the Latin word for time.)
Infinitive (base form): to study, to go
Present tense: Erin studies at the university. Erin goes to class.
Present participle: Erin is studying at the university. Erin is going to class.
Past tense: Erin studied at the university. Erin went to class.
Past participle: Erin has studied at the university. Erin has gone to class.
Gerund (noun form): Studying is hard work. Going to class is interesting.
English verbs have other forms that are important to use correctly.
Progressive forms (compound verbs with -ing show the continuation of an action, rather than its completion:
Present: I am walking Present Perfect: I have been walking Past: I was walking Past Perfect: I had been walking Future: I will be walking Future Perfect: I will have been walking
Present: I could walk Past: I could have been walking
Present: I do walk Past: I did walk
This last is a very important form to understand if English is your second language. For example, English-speakers almost always say or write, "How often do you walk to the store?" rather than "How often walk you to the store?," which sound very unnatural to the native speaker's ear.