For questions 7–15, rewrite each sentence, adding commas where needed.

(7) While I was researching skateboards I found an interesting old newspaper story.

(8) In the June 29 1999 edition of the Mercury there was an article on skateboarding.

(9) Some scientists in San Francisco California filmed skateboarders in action.

(10) As they studied the film they noticed some rules of science at work.

(11) To be a good skater you have to follow safety rules.

(12) Yes skateboarding is not only a sport but a science.

(13) When you balance on a skateboard there are three forces at work.

(14) The weight of the rider is one force and gravity is another.

(15) Pushing up against the skateboard the ground also acts as a force.

My answers:

7) While I was researching skate boards, I found an interesting old newspaper story.
8) In the June 29, 1999 edition of the Mercury, there was an article on skateboarding.
9) Some scientists in San Francisco, California filmed skateboarders in action.
10) As they studied the film, they noticed some rules of science at work.
11) To be a good skater, you have to follow safety rules.
12) Yes, skateboarding is not only a sport, but a science.
13) When you balance on a skateboard, there are three forces at work.
14) The weight of the rider is one force, and gravity is another.
15) Pushing up against the skateboard, the ground also acts a force.

8. One more comma is needed.

9. One more comma is needed.

The rest are correct.

8. there needs to be a comma in front of "Edition?"

9. Where should i put the comma? after california

Yes ... for both.

Thank you.

You're welcome.

(7) While I was researching skateboards, I found an interesting old newspaper story.

To add a comma in this sentence, we need to separate the dependent clause "While I was researching skateboards" from the main clause "I found an interesting old newspaper story."

(8) In the June 29, 1999 edition of the Mercury, there was an article on skateboarding.

To add commas in this sentence, we need to separate the date "June 29, 1999" from the rest of the sentence using a pair of commas.

(9) Some scientists in San Francisco, California, filmed skateboarders in action.

To add commas in this sentence, we need to separate the city "San Francisco" and state "California" from the rest of the sentence using a pair of commas.

(10) As they studied the film, they noticed some rules of science at work.

To add a comma in this sentence, we need to separate the introductory phrase "As they studied the film" from the main clause "they noticed some rules of science at work."

(11) To be a good skater, you have to follow safety rules.

To add a comma in this sentence, we need to separate the introductory infinitive phrase "To be a good skater" from the main clause "you have to follow safety rules."

(12) Yes, skateboarding is not only a sport but a science.

To add a comma in this sentence, we need to separate the introductory interjection "Yes" from the main clause "skateboarding is not only a sport but a science."

(13) When you balance on a skateboard, there are three forces at work.

To add a comma in this sentence, we need to separate the dependent clause "When you balance on a skateboard" from the main clause "there are three forces at work."

(14) The weight of the rider is one force, and gravity is another.

To add commas in this sentence, we need to separate the items in a list, specifically the phrases "The weight of the rider" and "gravity," using commas.

(15) Pushing up against the skateboard, the ground also acts as a force.

To add a comma in this sentence, we need to separate the prepositional phrase "Pushing up against the skateboard" from the main clause "the ground also acts as a force."