Today, I read this sentence in one of the essays I graded. "We musted understand the whole story before judging others." In fact, I have not seen "musted" since 2006 when I graded a short essay during the CRE Preparatory Written Examination course.
"Can, could, may, might, must, shall, should, ought to and will" are modal verbs in English grammar. Modal verbs are used to express the writer's or speaker's attitude to what he or she is writing or saying. There are three main categories of meaning.
Modal verbs do not have tenses like other regular / irregular verbs. So, "musted" does not exist at all. However, most modal verbs have other forms that allow us to express the same ideas in the past, present and future time.
Inspired by the "musted" in the essay I graded, let us use must in a different structure.
I must call my brother this morning.
I was late for the German class because I had to call my brother yesterday morning.,
The function of "must" and "had to" here are to show necessity; present and past.
Let's go back to the essay of that student, instead of using "musted" , we can say,
"We must have understood the whole story before judging others."
In this context, the function of "must" is to show probability or to make a logical assumption.
But then, not all modal verbs have past forms. For instance, most students of mine consider "would" as the past tense of "will" . Let"s look at this example,
Will you help me with my math homework?
Would you help me with my math homework?
"Would" is not the past form of "will" as both here refer to asking for a favour politely.
When I lived in Sai Wan Ho, I would eat Cantonese dim sum every other day.
"Would" here refers to a repeated action in the past and it has nothing to do with "will" .
We are always ready to learn although we do not always being taught. We can always learn about the usage of modal verbs and their alternative forms in our everyday life. As Albert Einstein said, "Wisdom is not a product of schooling but of the lifelong attempt to acquire it.”