Reported Speech Transformation:
In reported speech, we convey what someone else said without using their exact words. To change the statement "My friend said the earth was round" into reported speech, we need to follow a few rules:

Identify the Reporting Verb:
The reporting verb in this sentence is "said."

Change Pronouns and Time Expressions:
When changing the sentence into reported speech, we need to adjust the pronouns and time expressions. In this case, "my friend" will become "his/her friend" and "was" may change to "is" depending on the context.

Change the Verb Tense:
Since the original statement is in the past tense, we need to shift the verb tense in reported speech. "Was" will change to "is" to reflect that the earth's round shape is a fact.

Change the Reporting Verb if Necessary:
If the reporting verb is in the past tense, it may need to be changed to a past perfect tense depending on the overall context of the conversation.
By following these rules, the sentence "My friend said the earth was round" can be transformed into reported speech as:
His/her friend mentioned that the earth is round.
This transformation maintains the essence of the original statement while accurately conveying the information shared by the friend. Reported speech allows us to share statements made by others in a clear and grammatically correct manner.