Negation is explained in detail in Chapter 15. There are two forms of negation in Lojban: contradictory and scalar negation. Contradictory negation expresses what is false, whereas scalar negation says that some alternative to what has been stated is true. A simple example is the difference between “John didn’t go to Paris” (contradictory negation) and “John went to (somewhere) other than Paris” (scalar negation).
Contradictory negation involving BAI cmavo is performed by appending “-nai” (of selma'o NAI) to the BAI. A common use of modals with “-nai” is to deny a causal relationship:
13.1) mi nelci do mu'inai le nu do nelci mi
I like you, but not because you like me.
Example 13.1 denies that the relationship between my liking you (which is asserted) and your liking me (which is not asserted) is one of motivation. Nothing is said about whether you like me or not, merely that that hypothetical liking is not the motivation for my liking you.
Scalar negation is achieved by prefixing “na'e” (of selma'o NAhE), or any of the other cmavo of NAhE, to the BAI cmavo.
13.2) le spati cu banro na'emu'i le nu
do djacu dunda fi le spati
The plant grows other-than-motivated-by the event-of
you water-give to the plant.
Example 13.2 says that the relationship between the plant’s growth and your watering it is not one of motivation: the plant is not motivated to grow, as plants are not something which can have motivation as a rule. Implicitly, some other relationship between watering and growth exists, but Example 13.2 doesn’t say what it is (presumably “ri'a”).
Note: Modals made with “fi'o” plus a selbri cannot be negated directly. The selbri can itself be negated either with contradictory or with scalar negation, however.