109. A participle may be used predicatively with a form of "esti", as
"Mi estas demandanta", I am asking, "La viro estas acxetanta", the man
is buying. Such combinations are called "compound tenses", in contrast
to the "simple" or "aoristic" tenses.
[Footnote: An aoristic tense consists of but one word (ending in
"-as", "-os", etc.) and expresses an act or state as a whole, without
specifying whether it is finished, still in progress, or yet begun.]
Compound tenses occur less often in Esperanto than in English, and an
aoristic Esperanto tense may often be translated by an English compound
tense, as "La birdoj flugas", the birds are flying. When used to
form a compound tense, the verb "esti" is called the "auxiliary verb".
No other verb is ever used as an auxiliary (a simpler method than in
English, which uses "be", "have", "do", "will", "shall", "would", etc.).