Yesterday evening, I went to the supermarket. I bought lots of delicious food and I spent a lot of money. When I arrived home at eight o'clock, I made a tasty supper and ate it all myself! Yum yum!
We use the simple past tense (I went / I bought / I spent / I arrived / I made / I ate) to talk about one completed action done at a definite time in the past. The simple past (I saw) is the simple present (I see) done, as its name tells you, in the past.
Look at this sentence.
Today, I play tennis at five o'clock but yesterday I played tennis at seven.
Two tenses in one sentence! Oh, no! Which is the simple past? Easy. It's the action done yesterday. Yesterday at seven is finished time, so we use the simple past tense. Phrases like last week, yesterday morning, in 2001, 10 years ago, then, two months ago, five minutes ago are all finished pin-points in the past so the verb that follows must be the simple past.
Most verbs are regular and end in -ed in the simple past tense. (they arrived/we watched/dad listened). Some verbs are irregular (to see / to eat/ to go) and you have to learn the simple past of these because you can't work it out. There will probably be a list of these irregular verbs at the back of your English textbook, but you will soon get used to using irregular simple past tenses because they are easy words to remember.