Canst – (verb) Nominative case second-person singular simple present form of ‘can’ when used with the pronoun thou or its relative form.
The word can is not a modern replacement for the word canst. If canst is considered archaic, then can is equally archaic. The reality is that canst is a ‘normal verb’, and can is a ‘modal verb’.
A normal verb expresses an action. A modal verb is an auxiliary verb that expresses necessity or possibility. English modal verbs include must, shall, will, should, would, can, could, may, and might.
God used both words in every dispensation. He used ‘canst’ 51 times in Scripture, and ‘can’ 235 times in Scripture.
See also Can, Thou