How do you use the simple present, simple progressive, and simple past tenses?

The tenses of verbs are the forms that indicate the time of action or existence.
THE SIMPLE PRESENT TENSE
This tense has three uses. (1)The present tense can indicate an action that happens or a condition that exists now (in present time), (2) or an action that occurs on a regular basis. (3) It can also be used to express a general truth.Here are examples of each of these three uses of the present tense:
I wonder why that student continues to sit there even though the class is over. [an action happening now]
The bus arrives at 7:30 a.m. each morning. [a repeated action]
Gold is always valuable. [expression of a general truth]
There is also another use of the present tense, which is called the historical present tense. It is sometimes used in historical writing in order to express past events and more frequently in poetry, fiction, and writing about literature. In journalism, especially in sports writing, the present is used to convey a sense of being there for the reader.
THE PROGRESSIVE FORM
(There is no simple progressive)Each of the six tenses has a progressive form that is used to express continuing action [action "in progress"]. To create the progressive forms, one must use the appropriate tense of the verb to be with the present participle of the verb. So, the time of the action is indicated with the auxiliary, or helping verb. 
In order to form the other progressive tenses, the helping verb to be is changed to the appropriate tense, but the participle remains the same.Here are examples of the six tenses in the progressive form:
PRESENT Mary is sitting in the library. Peter and Joe are sitting in the gym.PAST       Mary was sittingFUTURE   Mary will be sitting
PRESENT PERFECT  Mary has been sitting. The girls have been sitting.PAST PERFECT        Mary had been sitting.FUTURE PERFECT    Mary will have been sitting.
THE SIMPLE PAST
The simple past is formed with the past form of the verb. This past tense is used in order to express an action or condition that was begun and completed in the past. The regular verbs form their past by adding -ed to the present form. Irregular verbs have a separate word, and the verb to be changes to was in the singular and were in the plural form.
Here are examples of the simple past (there is only one verb used):
The baby wanted his pacifier.The meeting went well. The swimmers all won medals. Note: The verb to be has two forms in its simple past: was [singular] and were [plural]. For example:The insurance agent was here yesterday. The repairmen were not here.
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