Learn Hardcore Hausa: Don Allah ku guji haɗari a titi. - Please avoid danger in the street.

In everyday speech it functions as a very common way to say please, appealing to someone “for God’s sake.” It sounds natural and polite in Hausa, not overly dramatic, and is used in many everyday requests and warnings, like in this sentence.

What is the role of ku in this sentence?

Ku is the second person plural subject pronoun (you plural) and also acts as the marker for a plural imperative here.

So:

ku guji = avoid! (said to more than one person)
It tells you the command is addressed to you (all), not just one person.

Without ku, the sentence would be ungrammatical in this form; Hausa imperatives usually need the appropriate subject pronoun (except in some special short forms).

How would this sentence change if I were speaking to only one person?

For a single person, you change ku to ka (to a man) or ki (to a woman):

To one man: Don Allah ka guji haɗari a titi.
To one woman: Don Allah ki guji haɗari a titi.
To several people: Don Allah ku guji haɗari a titi.