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1. Should this be "the regular and irregular past tense" or "regular and irregular past tenses"? They will learn to talk about activities using the regular and irregular past tense of "to be".--
There is no regular past tense of 'to be'. The correct sentence is 'They will learn to talk about activities using the past tense of "to be" '.2. What grammatical function does the word "opened" have? Could it be an adjective (with a present perfect tense)? He acts as if he is a blind person who has just had his eyes opened.
-- 'Opened' is the past participle for the present perfect coercive verb 'to have (something) opened (by someone)'.
Sorry but could we place an adverb like "just" ( I think it is an adverb") before the verb?--
Adverbs are generally placed before a simple verb or after the first unit of a verb phrase:
He often acts as if he is a blind person who has just had his eyes opened. .