Grammatically, every use of tenses seen so far is exactly paralleled by some use of modals as explained in Chapter 9. Modals and tenses alike can be followed by sumti, can appear before the selbri, can be used in pure and mixed connections, can participate in JAI conversions. The parallelism is perfect. However, there is a deep difference in the semantics of tense constructs and modal constructs, grounded in historical differences between the two forms. Originally, modals and tenses were utterly different things in earlier versions of Loglan; only in Lojban have they become grammatically interchangeable. And even now, differences in semantics continue to be maintained.
The core distinction is that whereas the modal bridi
23.1) mi nelci do mu'i le nu do nelci mi
I like you with-motivation the event-of you like me.
I like you because you like me.
places the “le nu” sumti in the x1 place of the gismu “mukti” (which underlies the modal “mu'i”), namely the motivating event, the tensed bridi
23.2) mi nelci do ba le nu do nelci mi
I like you after the event-of you like me.
I like you after you like me.
places the “le nu” sumti in the x2 place of the gismu “balvi” (which underlies the tense “ba”), namely the point of reference for the future tense. Paraphrases of Example 23.1 and Example 23.2, employing the brivla “mukti” and “balvi” explicitly, would be:
23.3) le nu do nelci mi cu mukti le nu mi nelci do
The event-of you like me motivates the event-of I like you.
Your liking me is the motive for my liking you.
and
23.4) le nu mi nelci do cu balvi le nu do nelci mi
The event-of I like you is after the event of you like me.
My liking you follows (in time) your liking me.
(Note that the paraphrase is not perfect due to the difference in what is claimed; Example 23.3 and Example 23.4 claim only the causal and temporal relationships between the events, not the existence of the events themselves.)
As a result, the afterthought sentence-connective forms of Example 23.1 and Example 23.2 are, respectively:
23.5) mi nelci do .imu'ibo do nelci mi
I like you. [That is] Because you like me.
23.6) do nelci mi .ibabo mi nelci do
You like me. Afterward, I like you.
In Example 23.5, the order of the two bridi “mi nelci do” and “do nelci mi” is the same as in Example 23.1. In Example 23.6, however, the order is reversed: the origin point “do nelci mi” physically appears before the future-time event “mi nelci do”. In both cases, the bridi characterizing the event in the x2 place appears before the bridi characterizing the event in the x1 place of “mukti” or “balvi”.
In forethought connections, however, the asymmetry between modals and tenses is not found. The forethought equivalents of Example 23.5 and Example 23.6 are
23.7) mu'igi do nelci mi gi mi nelci do
Because you like me, I like you.
and
23.8) bagi do nelci mi gi mi nelci do
After you like me, I like you.
respectively.
The following modal sentence schemata (where X and Y represent sentences) all have the same meaning:
X .i BAI bo Y
BAI gi Y gi X
X BAI le nu Y
whereas the following tensed sentence schemata also have the same meaning:
X .i TENSE bo Y
TENSE gi X gi Y
Y TENSE le nu X
neglecting the question of what is claimed. In the modal sentence schemata, the modal tag is always followed by Y, the sentence representing the event in the x1 place of the gismu that underlies the BAI. In the tensed sentences, no such simple rule exists.