The error is in third part. By using ‘not’ after ‘unless’, both the negatives cancel each other. So, the correct usage will be ‘unless he reports for duty immediately’. Since “unless” is negative it should be
followed by affirmation, like “unless he reports for duty”. Incase “unless” was absent, we could have written “if he does not report” and expressed the same meaning.

The error is in first part. ‘Considering’ is a preposition that relates the facts to the principal and hence does not need to be followed by another preposition ‘about’. A preposition like ‘considering’ should be followed by a pronoun like ‘these’ in this case. The use of about is unnecessary and shall be omitted. The sentence indicates that the principal offered him a seat only after considering certain facts.

Error is in third part. Here, “there” is an adverb that qualifies the verb ‘is’. The position of ‘there’ in the sentence is incorrect. The correct usage should be ‘there is no trouble’. It indicates the absence of any trouble in a country.

Error is in second part. Returning means coming back to place from where you began. So, the use of back after returned becomes redundant. Without using back, the meaning remains the same. We can omit ‘back’ in part b, and use ‘returned home.’

Error is in third part. Since “doing” was used in second part it should be followed by “doing” in third part as well instead of using “done”. Such repetitive figure of speech is used in
English grammar to create emphasis. This sentence indicates that a thing that is worth doing is also worthy of doing well. Here worth is the noun that must be followed by the verb doing.

The error is in second part. In the given sentence two events occur one after the other. ‘He left’ before ‘we reached’. So, the event that was completed before the occurrence of the next event shall be expressed in past perfect tense. Therefore, the correct form of verb will be “had left”.

Error is in third part. By writing ‘who’ in the second part it has already been made clear that the sentence talks about ‘a man’. The use of ‘he’ becomes redundant or repetitive. By omitting ‘he’ the sentence becomes grammatically correct.

The error is in first part. If we start the sentence with being, it means that we were rainy which is incorrect. The sentence indicates that the day was Rainy, and the day will be denoted by ‘It’. So ‘It being a rainy day’ will be the correct idiomatic expression. Otherwise the meaning of the sentence changes.