I forgot to include the following few sentences. Thank you very much.

1) If it hadn't been for (the action of) one brave woman, the boycot wouldn't have occurred.
2) I'm doubtful whether or not to stay with the same family as two of my classmates. (to share a room with two of my classmates)
3) Plastic bags have been replaced by 100% biodegradable bags and by paper shopping bags.
4) At the check-out counter you can either buy a biodegradable bag or a paper shopping bag. Most people reuse the same bag several times so that they can save money and protect the environment.
5) Can you tell me how high is the rate for a one-hour conversation class?

1. boycott (spelling)

2. Use "To share a room..." -- the other phrasing is not clear.

3. Remove the second "by"

4. Comma needed

5. ...how high the rate is...

Sure! Here are the explanations for the sentences you provided:

1) To determine the condition under which the boycott occurred, you can rewrite the sentence using a second conditional structure: "If one brave woman had not taken action, the boycott would not have occurred." The second conditional is used to express a hypothetical situation in the past and its imaginary result.

2) To clarify whether you should stay with the same family as your classmates, you can rephrase the sentence as follows: "I am unsure whether or not I should share a room with my two classmates." This sentence expresses uncertainty about the decision.

3) To explain the replacement of plastic bags, you can modify the sentence: "Plastic bags have been replaced by 100% biodegradable bags and paper shopping bags." This revised sentence explains the alternative options that have replaced plastic bags.

4) To explain the choices available at the check-out counter, you can rephrase the sentence: "At the check-out counter, you have the option to purchase either a biodegradable bag or a paper shopping bag. Many people reuse the same bag multiple times to save money and protect the environment." This revised sentence explains the two choices and provides additional information about the reasons for reusing the bags.

5) To ask about the rate for a one-hour conversation class, you can modify the sentence: "Can you please inform me of the cost for a one-hour conversation class?" This revised sentence makes the request for information clear.