Tenses are forms of a verb that show the time, continuance or completion of an action or a state that is expressed in connection with the moment at which a statement is made about it. There are three main tenses: the present, the past, and the future. For each of the tenses, there are four aspects: simple, continuous or progressive, perfect and perfect continuous or perfect progressive.

The present tense is used to refer to something that happens or exists now. The present tense is also called the present simple or simple present.

The past tense is used to refer to something that happened or existed in the past. The past tense is also called the past simple or simple past.

The future tense refers to something that hasn’t happened at the time of speaking. The future tense makes use of the words will and shall.

The continuous aspect indicates an unfinished action or state of being at the time being referred to. It is formed using the relevant tense of the auxiliary verb to be and the present participle of the main verb.

The perfect aspect refers to a completed action in the present, past or future. It is formed using the combination of the relevant tense of the auxiliary verb have and the past participle of the main verb.

The perfect continuous aspect combines both aspects of continuous and perfect.  It refers to an action that has continued up to the present, up to a time in the past, or up to a time in the future. 

The three verb tenses and four verb aspects make possible the twelve combinations of tense and aspect, as follows: