We use simple past for
- A completed action in the past. e.g. I walked to work yesterday.
- A repeated past action in the past. e.g. I studied hard in school.
- A sequence of events that happened in order. e.g. I walked home then a talked to a friend.
- Last night/month/year
- Ten days ago
- In 1975
If you see one of these keywords, it indicates that the sentence is in the past.
For more on verb tenses, check out the verb tense overview, or for some practice check out Englishpage.com