**Past form of the verbs "Teach," "Watch," "Sell," and "Explain"**

**Teach:**
The past form of the verb "teach" is "taught." It is used to describe an action that occurred in the past. Here are a few examples of sentences using the past form of "teach":

- My teacher taught us about the solar system yesterday.
- She taught English at the university for ten years.
- The experienced coach taught the team some new strategies.

**Watch:**
The past form of the verb "watch" is "watched." It is used to indicate that an action of observing or looking at something happened in the past. Here are some examples of sentences using the past form of "watch":

- We watched an interesting movie last night.
- He watched the birds flying in the sky for hours.
- They watched the sunset from the top of the mountain.

**Sell:**
The past form of the verb "sell" is "sold." It is used to describe an action of exchanging goods or services for money in the past. Here are a few examples of sentences using the past form of "sell":

- The store sold all its products during the holiday sale.
- She sold her old car and bought a new one.
- They sold their house and moved to a different city.

**Explain:**
The past form of the verb "explain" is "explained." It is used to describe an action of making something clear or understandable to someone in the past. Here are some examples of sentences using the past form of "explain":

- The teacher explained the math problem step by step.
- He explained the rules of the game to the new players.
- She explained her reasons for being late to her boss.

In conclusion, the past forms of the verbs "teach," "watch," "sell," and "explain" are "taught," "watched," "sold," and "explained" respectively. These past forms are used to describe actions that happened in the past.