complete the sentence.

My answers are with capital letter

IF YOU WERE TO GO BACK 4500 years in Britain, YOU MIGHT WITNESS the rise of Stonehenge.
WE’RE NOT FOR hundreds of people helping, COULD NOT HAVE BEEN built. IF IT HADN’T BEEN for the highly organized effort of these people using only simple tools,WE WOULD NOT HAVE the historic treasure today.
IF I COULD go back to one time to participate in events, I WOULD CHOOSE to work along with them to raise the blue stone megaliths.

"We're not for hundreds of people helping, could not have been built" makes no sense at all. The others look fine. You need some nouns and/or pronouns. And I'm glad we are not for hundreds of people helping, but why are we not for it?

Complete the sentence:

IF I WENT BACK in time to Britain 4500 years ago, I WOULD LIKELY WITNESS the rise of Stonehenge.

Explanation:
To complete the sentence, we need to change the verb "were" to "went" to match the verb tense of the sentence. Additionally, we should replace the phrase "might witness" with "would likely witness" for better clarity and accuracy.