Complete with the past simple.

1. There
fifty people at the party on Saturday. (to be)

2. She
her house many years ago. (to sell)

3. They
in the lake last summer. (to swim)

1. There were fifty people at the party on Saturday. (to be)
- In this sentence, we are talking about an event that happened in the past, specifically on Saturday. We use the past simple tense of the verb "to be" which is "were" to indicate that there were fifty people at the party on that day.

2. She sold her house many years ago. (to sell)
- In this sentence, we are talking about an action that happened in the past, specifically when she sold her house. We use the past simple tense of the verb "to sell" which is "sold" to indicate that she completed the action of selling her house in the past.

3. They swam in the lake last summer. (to swim)
- In this sentence, we are talking about an activity that took place in the past, specifically last summer. We use the past simple tense of the verb "to swim" which is "swam" to indicate that they engaged in the activity of swimming in the lake during that time.

Overall, when we want to talk about actions or events that happened in the past, we use the past simple tense of verbs. It is important to remember the specific conjugation of each verb in the past simple tense for different subjects, as it can vary.