Final answer:
To rewrite Mathew's statement in reported speech, change the sentence to past tense and include 'that': "Mathew complained that it had been a bad day for him."
Step-by-step explanation:
When Mathew says "this day is a bad day for me," and you are asked to begin: Mathew complained that, you are tasked with turning his direct speech into reported speech. In reported speech, you need to make some changes to the original sentence to fit the reporting clause. Remember to backshift the tense from present to past if the reporting verb is in the past tense, which it is in this case. Also, the word 'that' is often used to introduce the reported clause. So, the reported speech version of Mathew's complaint would be: "Mathew complained that it had been a bad day for him."