***** NOT A TEACHER *****
(1) May I simplify your sentences for easier analysis?
(a) "She saw the leaf fall."
(b) "She waited for the leaf to fall."
(2) I have checked my books, and I may have an explanation.
(3) First, may I most gently remind you that in sentence 1a, "fall" is not called a base form. It is considered a bare infinitive (that
is, one without "to").
(4) My books (which will be credited at the end of this post) tell me that after certain verbs of "sensory perception," native
speakers have decided to use the bare infinitive:
I heard the leaf fall.
I watched the leaf fall.
I saw the leaf fall.
(P.S. Sorry to make things even more confusing, but "I saw the leaf fall" = I saw it hit the ground; "I saw the leaf falling" = I did
NOT see it actually hit the ground. Maybe I had turned my head away for some reason.)
(5) If the verb is NOT one of "sensory perception," then we use the to-infinitive:
I expected the leaf to fall.
I wanted the leaf to fall.
I waited for the leaf to fall.
(NOTE: "Wait for" is sometimes called a so-called "prepositional verb," for the preposition "for" most often collocates with "wait.")
If I have given you any accurate information, all credit is due to:
The 1983 edition of The Grammar Book by Mesdames Celce-Murcia and Larsen-Freeman, page 484.
The second edition of Descriptive English Grammar by Professors Homer C. House and Susan E. Harman, page 330.
Various websites on Google.
HAVE A NICE DAY!