Example: Your tuition could be completely covered.
Question: Why is this sentence correct? I thought with modal, you must always use base form.
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Be is the verb in base form just after could. Covered (past participle) is used here after be as the sentence is in the passive voice. In the passive voice, the main verbs are written as
'to be' verb in the correct tense + past participle of main verb
The 'performer of the verb' in this form is either given less importance or is omitted completely. Here the speaker has omitted the 'performer of the verb'. In the active voice, the sentence could have been written in this way:
The student loan could completely cover your tuition.