4. Temporal tenses: PU and ZI

   pu  PU  past
    ca  PU  present
    ba  PU  future

    zi  ZI  short time distance
    za  ZI  medium time distance
    zu  ZI  long time distance

4.1)  le nanmu pu batci le gerku
    The man [past] bites the dog.
    The man bit the dog.

Lojban recognizes three temporal directions: ``pu'' for the past, ``ca'' for the present, and ``ba'' for the future. (Etymologically, these derive from the corresponding gismu ``purci'', ``cabna'', and ``balvi''. See Section 23 for an explanation of the exact relationship between the cmavo and the gismu.) There are many more spatial directions, since there are FAhA cmavo for both absolute and relative directions as well as ``direction-like relationships'' like ``surrounding'', ``within'', ``touching'', etc. (See Section 27 for a complete list.) But there are really only two directions in time: forward and backward, toward the future and toward the past. Why, then, are there three cmavo of selma'o PU?

The reason is that tense is subjective: human beings perceive space and time in a way that does not necessarily agree with objective measurements. We have a sense of ``now'' which includes part of the objective past and part of the objective future, and so we naturally segment the time line into three parts. The Lojban design recognizes this human reality by providing a separate time-direction cmavo for the ``zero direction'', Similarly, there is a FAhA cmavo for the zero space direction: ``bu'u'', which means something like ``coinciding''.

(Technical note for readers conversant with relativity theory: The Lojban time tenses reflect time as seen by the speaker, who is assumed to be a ``point-like observer'' in the relativistic sense: they do not say anything about physical relationships of relativistic interval, still less about implicit causality. The nature of tense is not only subjective but also observer-based.)

Here are some examples of temporal tenses:

4.2)  le nanmu puzi batci le gerku
    The man [past] [short distance] bites the dog.
    A short time ago, the man bit the dog.

4.3)   le nanmu pu pu batci le gerku
    The man [past] [past] bites the dog.
    Earlier than an earlier time than now,
        the man bit the dog.
    The man had bitten the dog.
    The man had been biting the dog.

4.4)   le nanmu ba puzi batci le gerku
    The man [future] [past] [short] bites the dog.
    Shortly earlier than some time later than now,
        the man will bite the dog.
    Soon before then, the man will have bitten the dog.
    The man will have just bitten the dog.
    The man will just have been biting the dog.

4.5)  le nanmu zi pu batci le gerku
    The man [short] [past] bites the dog.
    Before a short time from or before now,
        the man bit or will bite the dog.

4.6)   le nanmu zu batci le gerku
    The man [long] bites the dog.
    A long time from or before now,
        the man will bite or bit the dog.

Because we can move in any direction in space, we are comfortable with the idea of events happening in an unspecified space direction (``nearby'' or ``far away''), but we live only from past to future, and the idea of an event which happens ``nearby in time'' is a peculiar one. Lojban provides lots of such possibilities that don't seem all that useful to English-speakers, even though you can put them together productively; this fact may be a limitation of English.

Finally, here are examples which combine temporal and spatial tense:

4.7)  le nanmu puzu vu batci le gerku
    The man [past] [long time] [long space] bites the dog.
    Long ago and far away, the man bit the dog.

Alternatively,

4.8)  le nanmu batci le gerku puzuvuku
    The man bites the dog [past] [long time] [long space].
    The man bit the dog long ago and far away.