How to add emphasis to a modal verb?

Solution 1:

You could add some additional words to convey the emphasis, or rephrase slightly:

Number 1:

He certainly can run fast.

He can definitely run fast.

Number 2:

Please do come in

Come in, please (more of a command, to provide the additional emphasis).

You may come in now (the emphasis on "may" in the example suggests that the listener wasn't allowed to come in before, but can now).

Number 3:

I really must brush my teeth.

I definitely have to brush my teeth.

Solution 2:

This is more difficult to do in writing than in speech. In speech you can make the verb, noun, or preposition slightly louder than normal. In my examples, the words in all capital letters denote the emphasis.

Verbs:

• He CAN run. (He is able to run, even if you doubt it)
• He can RUN. (He can run very quickly)
• You MAY come in. (You are permitted to come in. Maybe you did not know if you were allowed to enter)
• I SHOULD brush my teeth. (I would prefer to brush my teeth right now. However, it is not an absolute necessity and I can engage in other activities first.)
• I should BRUSH my teeth. (Instead of letting them rot)

Nouns/Prepositions:

• You may come IN. (Said this way, the sentence expresses irritation. Perhaps you did not enter quickly enough)
• I should brush my TEETH. (As opposed to my hair)

The above emphasis is used the way I have described it in spoken American English in the regions in which I have lived. There may be regional variation of which I am unaware.

For written English, I agree with Steve Melnikoff's suggestion that you find adjectives or adverbs to get the point across more clearly.