You may call someone's attention to the fact that you are addressing them by using doi followed by their name. The sentence

means “Oh, John, I'm talking to you”. It also has the effect of setting the value of do; do now refers to “John” until it is changed in some way in the conversation. Note that Example 2.55 is not a bridi, but it is a legitimate Lojban sentence nevertheless; it is known as a “vocative phrase”.

Other cmavo can be used instead of doi in a vocative phrase, with a different significance. For example, the cmavo coi means “hello” and co'o means “good-bye”. Either word may stand alone, they may follow one another, or either may be followed by a pause and a name. (Vocative phrases with doi do not need a pause before the name.)

Commands are expressed in Lojban by a simple variation of the main bridi structure. If you say

you are simply making a statement of fact. In order to issue a command in Lojban, substitute the word ko for do. The bridi

instructs the listener to do whatever is necessary to make Example 2.58 true; it means “Talk!” Other examples:

The ko need not be in the x1 place, but rather can occur anywhere a sumti is allowed, leading to possible Lojban commands that are very unlike English commands:

The cmavo ko can fill any appropriate sumti place, and can be used as often as is appropriate for the selbri:

and

both mean “You take care of you” and “Be taken care of by you”, or to put it colloquially, “Take care of yourself”.