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Esperanto is a language with very rich word building. There is a large system of affixes. In addition, there are no phonologic alternations (compare kantas – kantis – kantinta – kanto with sing – sang – sung – song.).
Elements of Esperanto can be classified into these categories:
1) Roots (radikoj):
patr – man, bon – good, ir – go
2) Affixes (afiksoj) – subset of roots with some specificity (see chapter 4.2)
ej – place, ism – a movement, iĝ – to become
3) Inflectional affixes or endings (finaĵoj) (described in chapter 3):
a) category endings: o – noun, a – adjective, e – adverb, i – infinitive verb
b) declensional endings: j – plural, n – accusative
c) conjugative endings: a – present, i – past, o – future, s – indicative, nt – active participle, t – passive participle, us – conditional, u – volitive
4) Primitive words (vortetoj) – a subset of roots that do not require any category ending to form a word. However, the ending is possible.
tro – too, too many, tri – three, vi – you, aŭ – or
fi – fie ◊ fia – disgusting, anstataŭ – instead of, anstataŭi – to substitute
All Esperanto roots (without primitive words) can be classified into three categories – nominal roots, adjectival roots and verbal roots. These categories are inherent to them:
• nominal roots: hom – human, martel – hammer, buter – butter, domo – house
• adjectival roots: bel – nice, bon – good, blu – blue
• verbal roots: kur – run, kapt – catch, dir – say
Suffixoids can be classified into these three categories too:
• nominal roots: ul – person, ej – place, il – tool, in – feminine
• adjectival roots: ebl – able, em – having tendency, end – necessary
• verbal roots: ig – to cause, iĝ – to become
The primitive words have different POS:
mi – I (pronoun), tiu – this (pron.), apenaŭ – scarcely (adverb), tre – very (adv.), kial – why, tra – through (preposition, sed – but (conjunction), ĉu – whether (particle)
Sometimes the category of the root is obvious, sometimes it is arbitrary set (komb – comb (v), bros – brush (n)).
If a category ending (o for a noun, a for an adjective and i for a verb) is added to the root of the same category, it does not change the meaning of it. In this case, the grammatical endings are redundant.
Theoretically, any root can be converted to any category just by assigning the ending of that category. The meaning of the result is depending on the category of the root:
1) Noun ending (o)
a) With adjectival roots – the name of the quality expressed by the root.
bela – beautiful ◊ belo – beauty, beautifulness
b) With verbal roots – the name of the process expressed by the root.
kuri – to run ◊ kuro – a run
2) Adjectival ending (a) with noun or verbal roots – the quality of or relation to the concept expressed by the root.
reĝo – king ◊ reĝa – royal
ami – to love ◊ ama – amatory
3) Verbal ending (i) with nominal or adjectival roots – action or state characterized by the concept expressed by the root.
martelo – hammer ◊ marteli – to hammer, to work with the hammer
bela – nice ◊ beli – to look nice
If the meaning cannot be expressed by endings, it is necessary to use suffixes. For example, the name of the process can be expressed simply by using the noun ending only with verbal root (kuri ◊ kuro). This is impossible with roots of another category (marteli – to hammer, martelo – a hammer, not working with hammer). In this case, it is possible to use suffix ado (martelado – working with hammer). Using this suffix followed by a noun ending after a verbal root is a redundancy, but it is used to stress the fact of the process. The quality can be expressed by the suffix eco (marteleco – the quality of being a hammer, beleco = belo). For more suffixes see chapters 4.2.1 and 4.2.2.
In the following text, the roots and affixes are very often showed with their category endings. The phrase “root domo” is an abbreviation for “nominal root dom.”
As an example of word derivation I quote some words derived from the root labori – to work:
laboro – work (n.)
labora – work (adj.)
labore – by a work
labor| isto – worker
labor| ist| ino – female worker
labor| ist| aro – labor (workers considered as a group)
labor| ego – grand work
labor| aĵo – the thing concerned with a work
labor| ebla – workable
labor| ejo – workshop, workplace
ek| labori – to start to work
labor| estro – the chief of the work
labor| ema – laborious
mal| labor| ema – lazy
labor| em| ulo – hard worker
labor| en| da – that has to be done
fi| laboro – disgusting work
labor| ilo – a tool for a work
re| labori – to do again, to start work again
labor| ulo – worker
sen| labor| ulo – unemployed person
labor| tago – work day
tag| laboro – the work for the day
 Because of the origin of the words, it is possible to look at some Esperanto words as having also some phonological or orthographical changes: agi, akto, reakcio – to act, action, reaction; inteligenta, intelekto – intelligent, intellect; etc. These forms are regarded as distinct roots. See also pseudoaffixes (4.2.5)
 See chapter 4.2
 Věra Barandovská: Esperanto pro samouky, p. 183 and Karel Kraft: Esperantsko-český slovník.