The past tense of a main verb shows what happened or what state of being existed earlier at a definite time. When you use the past tense, the activity or state of being is over, finished, done (I drank the water, He was happy).
A regular verb usually forms its past tense by adding ‑ed (I decide, I decided). An irregular verb doesn’t add ‑ed. It might change an internal vowel (I drink, I drank).
Every one-word verb has a one-word past tense. When you move beyond the present tense and the past tense, however, you'll need to use auxiliary verbs to form other tenses (I will study, I have studied, and so on).
If the time portrayed is indefinite or if the action or state of being continues to the present, then the use of the past tense is improper. Instead, you should use the present perfect. Consider the following incorrect use of the past tense:
Instead, use the present perfect:
Or suppose you are at a friend’s party. You would not say in the past tense, “We enjoyed your party.” The enjoyment continues to the present. Thus, you would use the present perfect: “We have enjoyed your party.”