Usually, modal verbs are taught all at once. Like this:

I can go to school. May I come in? You must work.

So, when the student encounters "You may work", they are sure to translate it as "You must work".

And then comes the Present Perfect. In a quick refresher course, almost unevitably the Present Perfect coincides with HAVE TO. It seems that the only way to understand it is to have previously understood it.

I used to teach functions of HAVE all at once, but now I don't. Instead, I got a placard of a handsome young man whose strong-musculed arms were covered with thick hair.

And I called him Mr Must. This cartoon originally came from a booklet of an Indian air company. The lady was the boy's mother and she was trying to persuade the cashier (a boy himself), that her boy was travelling free because he was under 12. I had been looking for a picture just like that when I came across this one.

In my classroom, though, he was standing alone, and there were messages which I used to shout with my fists clinched, and the group repeated in choir:

YOU MUST WORK!!!

YOU HAVE TO WORK!!!

YOU SHOULD WORK!!!

You are to work!  - this one was in smaller letters, and pronounced mildly.

('used to' - because I am not teaching English this year, I am working as a school psychologist)

There is never a TO after a real modal verb, with the exception of OUGHT, which later appeared on a separate little sheet.

But there is a TO after a 'modal expression' like HAVE TO or BE ALLOWED TO.

Now, if somebody was having trouble with MAY, they could look at Mr Must's sayings and find out whether there was a MAY there. MAY wasn't there, so it had to be translated like CAN. If somebody was mixing up I HAVE WRITTEN with I HAVE TO WRITE, we read Mr Must's messages, and I pointed out, that HAVE TO requires the first form.

It looked childish. I told my 18-year-olds it was good to be childish.

But I needed another picture, of Mr Can, the bicyclist who would say

I CAN RIDE A BICYCLE.

I MAY RIDE A BICYCLE.

I AM ABLE TO RIDE A BICYCLE.

I AM ALLOWED TO RIDE A BICYCLE.

And one of my students drew him for me on a large sheet which was placed high, right under the ceiling, to the left of the blackboard. Later a piece of paper with COULD appeared. Students tended to know COULD.

Mr Must settled on the right-hand wall, almost in the opposite corner. Now I could ask the students, on what wall a particular modal verb or expression was.

I also made an 'official document' (with Mr Can's photo and a stamp) allowing Mr Can to ride his bicycle. It was full of phrases such as WE ALLOW, AND SO HE IS ALLOWED... WE PERMIT, anв so forth. It wasn't actively used, but could be seen on my walls.

And the idea of my little presentation is:

Avoid teaching CAN and MUST at the same time. (That is, topics like "Modals" and "Functions of HAVE")

CAN and MAY can be taught together if, for your purposes, they are translated identically. Here we can talk a little about Passive voice, as we write down 'be allowed to' which is considered the official 'modal expression' for MAY - in my institution.

Don't tell the general public that SHOULD can be used with the Perfect Infinitive. I did it once. I shouldn't have done it. The result was the whole group saying *I SHOULD HAVE TO. Fortunately, they didn't seem to remember anything in a week.

The cry over spilled milk, as I often called SHOULD HAVE DONE, should take place privately - between you and the few who want to know more and stay after the lesson.

Everything was written in coloured pens. There was no computer technology used except for reverting Mr Must's image so he would look in the opposite direction. I am often told that serious papers are printed in serious fonts, not written in hand. Yes, I fully agree - if they are not intended for actual reading.

Thank you for your attention! I hope you will enjoy teaching modals this way.