Indefinite: io, anything, something.
Negative: nenio, nothing.

234. A pronoun (not personal) in predicate or relative relation to a pronoun ending in -o must itself be of the same series:

Kio estas ĉi tio, kion vi diras? What is this, which you say?
Ŝi vidis tion, kio ĵus okazis, she saw that which just occurred.
Ĉio ĉi, kion vi vidas, estas farita de ili, everything here (all this), which you see, was done by them.
Li havas ion por vi, sed nenion por mi, he has something for you, but nothing for me.

CORRELATIVE WORDS.

235. Pronouns, adjectives and adverbs, which are related to each other as corresponding demonstratives, interrogatives, relatives, etc., are called correlatives. In Esperanto the correlative system is more complete than in any other language, and may be summarized as follows:

DemonstrativeInterrogative
and
Relative		 Distributive Indefinite Negative
tio (233)
that (thing) 		kio (233)
what, which 		ĉio (233)
everything 		io (233)
anything 		nenio (233)
nothing
tiu (56)
that (one) 		kiu (146)
who, which 		ĉiu (173)
every, each 		iu (203)
any (one) 		neniu (220)
no (one)
ties (62)
that one's 		kies (147)
whose 		ĉies (174)
every one's 		ies (204)
any one's 		nenies (221)
no one's
tia (65)
that kind of 		kia (150)
what kind of 		ĉia (177)
every kind 		ia (208)
any kind 		nenia (224)
no kind of
tie (68)
there 		kie (151)
where 		ĉie (182)
everywhere 		ie (209)
anywhere 		nenie (225)
nowhere
tiam (73)
then 		kiam (155)
when 		ĉiam (187)
always 		iam (212)
any time 		neniam (226)
never
tial (78)
therefore,
so 		kial (129)
wherefore,
why 		ĉial (188)
for every
reason 		ial (213)
for any
reason 		nenial (229)
for no
reason
tiel (88), (156),
thus, so 		kiel (156)
how, as 		ĉiel (193)
every way 		iel (216)
any way 		neniel (230)
in no way
tiom (104)
that much,
so much 		kiom (164)
how much,
as 		ĉiom (194)
all, the
whole of 		iom (217)
some, any
amount 		neniom (231)
none, no
quantity

THE USE OF AJN.

236. The word ajn may be placed after any interrogative-relative or indefinite correlative word, to give a generalizing sense. In order to avoid confusion with the accusative plural ending, ajn is never attached to the correlative which it follows:

kio ajn, whatever.
kies ajn, whosesoever.
kie ajn, wherever. 		kiam ajn, whenever.
kiom ajn, however much.
ia ajn, any kind whatever.