I am not a professional teacher but this ia a simple question which I can answer. The ‘ing’ or present participle form of a verb is used as gerund (or verbal noun). The present participle can not be directly used as verb in a sentence. The group of words "I walking to the store" is an incomplete and ungrammatical sentence, while word group "I am walking to the store" is a complete sentence. The words removing, carrying, arriving, washing and polishing used here, all function as gerunds (rather gerund phrases)? The continuous tense forms need the help of verb "be" to make a complete sentence as shown in the above example.
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