In simple terms, things can have already happened (past tense), can be happening now or continuously happening (present tense), or can yet to have happened (future tense). A verb’s tense denotes this. There are six basic tenses (two for each of the present, past, and future):
Usually you should maintain the same verb tense within a piece of writing and avoid inadvertent slips into a different tense. For example:
The club had a strict policy regarding the introduction of visitors who are allowed to use the club on weekdays.
Would be better written:
The club had a strict policy regarding the introduction of visitors who were allowed to use the club on weekdays.
The club has a strict policy regarding the introduction of visitors who are allowed to use the club on weekdays.
Sometimes you may wish to show things that happened in different time frames. For example:
Sam and his mother developed a close bond and this keeps them close.
The company struggled for many years but they were starting to turn the corner.
Both the above examples reflect a shift in time from the past to the present.
The past tense form of an irregular verbs does not end in -ed.