Okay, let's complete the sentences using the Past Continuous tense. Remember, the Past Continuous is formed with 'was/were + verb-ing'.
A: What were you doing last night at 8 o'clock?
Explanation: Since the subject is 'you' (plural/formal singular), we use 'were'. Then we add the -ing form to the verb 'do'.
B: I was chatting on the Internet.
Explanation: The subject is 'I', so we use 'was'. Then we add the -ing form to the verb 'chat'. Note that we double the 't' before adding -ing because the word follows the rule of consonant-vowel-consonant.
A: What were your friends doing at 6 p.m. yesterday?
Explanation: The subject is 'your friends' (plural), so we use 'were'. Then we add the -ing form to the verb 'do'.
B: They were cooking dinner.
Explanation: The subject is 'they' (plural), so we use 'were'. Then we add the -ing form to the verb 'cook'.
A: Was your brother watching TV at 10 a.m. this morning?
Explanation: This question is already correctly formed in the Past Continuous.
B: No, he wasn't watching TV. He was reading his favourite magazine.
Explanation: The subject is 'he' (singular), so we use 'was'. To form the negative, we add 'not' after 'was', resulting in 'wasn't'. Then we add the -ing form to the verb 'watch'. In the second part, we again use 'was' with 'he' and add the -ing form to the verb 'read'.