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irregular verb underlie
The
future perfect continuous expresses the duration of activities that extend until a certain point in time in the future. While this tense is hardly ever used in spoken English, it is still useful to know and understand it.
For a basic understanding of how this tense works, you need to be familiar with two very similar tenses: the past perfect continuous and the present perfect continuous. Both of these tenses are formed in the same way as the future perfect continuous tense.
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