Question

The modern word for cans’t.

Answer 1

Answer:

According to Wiktionary, 'canst' was a simple singular present tense of the term ‘can’, used by second person. It was often used by many poets and writers, like Shakespeare, Milton, etc., in the 16-17th century.

It was generally associated with the pronoun thou.  

Some examples which describe the usage of this word are presented below:

Canst thou show us to any house where we may have Shelter and Lodging tonight?

Thomas Potts, The Wonderfull Discoverie of Witches in the Countie of Lancaster

Canst thou bind the sweet influences of Pleiades, or loose the bands of Orion

Job 3831, The Bible

Explanation:

Answer 2

We can refer "cans't" by a modern word i.e. "can".

This is actually an old or archaic form of word  used to denote "can" form in any sentence. So, other words, it is a way of saying that, this word stands as an archaic form of word in its second person singular form, in present tense, to express that something "can' be done, can happen etc.

So it can refer about persons wanting to do something, or things that can happen at a point of time.

General example:

What services canst thou do"? - This was a question asked in King Lear.

So this word can be used as biblical verb, are as an archaic word.