A brief explanation of terminology before continuing:
A root is the root of the word, the basic building block which contains, in essence, the meaning or idea being worked with in the particular word. Most roots are inherently nominal, verbal, or adjectival (the remainder being numerals, prepositions, etc.)
A stem consists of the root (or roots, in the case of compound words) plus any prefixes suffixes added to modify the meaning of the root(s). In other words, the word minus the ending.
An ending is the termination of the word, which usually gives grammatical information, ie. what part of speech the word is, how it fits into the sentence, and so on. Sometimes there may be more than one, eg. plural nouns end in -es, the -e showing the word is a noun and the -s showing it to be plural.
A complete word, then, consists of the stem plus any ending(s).
There is no indefinite article, but the numeral un ("one") can be used to specify a single object or being.
The definite article is li, which is invariable. (It is actually the adjectival form of the third person pronoun, see below.)
The basic noun ending is -e. All nouns from nominal roots take this ending. This ending is added to adjectival stems to form nouns denoting persons, eg. from the adjective boni, we derive bone, "a good person (or other being)". Plurals are formed by adding -s.
Nouns denoting living beings are epicene, ie. show no distinction for gender. In order to give this information, the -e is changed to an -o to denote male beings, or to an -a to denote female beings. Plurals for these are formed, as with other nouns, by adding -s.
There are two additional noun endings, used when making nouns from adjectival stems. As noted above, adding -e to an adjectival stem creates an epicene noun. Adding -u creates concrete neuter nouns, ie. nouns denoting a physical, concrete object, eg. bonu, "a good thing/object". Adding -um creates abstract neuter nouns, eg. bonum, "good" (as in Li bonum de disi situatione es ke..., "The good (thing) of this situation is that..."); from fakti "true, factual" we get faktum "a fact". When added to adjectives of nationality, this ending creates nouns denoting the language spoken, eg. from angli "English" we get li anglum "the English language".
There are 2 further endings to consider. Adding -(e)n to a noun creates the Genitive or possessive case, eg. li mediken hause "the doctor's house", li medikesen hauses "the doctors' houses". It can be seen that this ending functions more or less as the "apostrophe-S" does in English. The other ending is -(e)m, which creates the Accusative case, ie. it indicates the direct object of the verb. This case is very rarely used, as word order normally suffices to indicate this. It is only used when ambiguity might otherwise arise, ie. when using unusual word order for effect, rhythm, etc.
There is a small number of neuter nouns which do not end in -e though they seemingly should according to the rules of the language, eg. piano, manu. Most of these are verbal nouns denoting the act of doing something, which also end in -o, eg. kanto "(the act of) singing" from kanta "(to) sing". There are even a few nouns which end in a consonant, eg. harem (though most of these are neuter abstracts in -um).
The third person pronouns act as if formed from the adjective li (the definite article), so the forms le and les are epicene. The masculine pronouns are therefore lo "he" and los "they (all male)"; the feminine pronouns are la "she" and las "they (all female)"; the neuter concrete pronouns are lu "it" and lus "they (all concrete objects)"; the neuter abstract pronoun is lum "it". An example of the usage of these different pronouns is given in the Novial Lexike:
There is also a third person reflexive pronoun se, as in lo prendad sen libre "he took his (own) book", as opposed to lo prendad lon libre "he took his (another guy's) book".
The adjectival ending is -i. This can be left off if the word is still easily pronounceable and no ambiguity will result, so eg. boni can be reduced to bon (but mikri cannot be reduced to mikr). Adjectives may take the -s, -n and -m endings if they are necessary and there is no noun to take them.
The general adverbial ending is -im, which is the most common adverbial ending (there are some additional less common adverbial endings with specific meanings, eg. -man forms adverbs of manner, -lok forms adverbs of place, -tem forms adverbs of time). Adverbs derived from prepositions take the ending -u, like inu "inside" from in "in", intru "between (them), inbetween" from inter. As seen in the latter example, prepositions ending in -er lose the -e- when the -u is added.
Comparison is done using the words plu "more", maxim "most", min "less", and minim "least".
Virtually all verbs in Novial end in one of the four vowels -a, -e, -i or -u, in descending order of frequency. The only exceptions I found were the verbs es "to be" and mus "to have to, must".
The basic verbal form is used as the infinitive, present tense, and imperative, though the infinitive may be enforced by the particle tu, eg. me have nulum tu fa "I have nothing to do".
The past tense is formed either by adding -(e)d or by using the auxiliary verb did, so "I did" is either me fad or me did fa. Either of these forms may be used; they are alternatives and there is no difference in meaning.
All other tenses are formed using auxiliary verbs. There are again two alternatives for the future tense, sal and ve, so "I will do" is either me sal fa or me ve fa. As with -d and did, there is no difference in meaning between the two. The past tense ending may be added to form the "sequence of tenses" found in the Germanic and Romance languages, eg. la dikted, ke la saled veni "she said that she would come" (ie. her words were me sal veni "I'll come").
The conditional if formed with the auxiliary vud, eg. me vud fa "I would do". Four verbs, es "be", deve "should, ought", pove "be able, can" and voli "want", have alternative inflected conditional forms esud, devud, povud and volud beside vud es, vud deve (?), vud pove and vud voli. I don't really understand devud as "would should" or "would ought to" doesn't make any sense to me. "Musud" beside vud mus "would have to" does make sense, but doesn't exist. My assumption is that devud was based on the conditional forms of the verb "have to" in the Romance languages (eg. French "je devrais", Spanish "deberķa"), which are used in the same sense as English "should, ought" in addition to "would have to". Perhaps Jespersen was thinking of dropping the verb mus and changing the meaning of deve to "have to", but what's given in the dictionary is mus = "have to, must, be obliged to" and deve = "should, ought to".
The perfect tenses are formed with the auxiliary ha, eg.
The present (active) participle is formed by adding -nt(i) to verbs ending in -a or -e, and -ent(i) to verbs ending in -i, -u or a consonant.
The past passive participle is formed by adding -t(i).
There are two types of passives in Novial, the "dynamic" passive which denotes an action, and the "stative" passive, which denotes a state resulting from a past action. The first is formed with the auxiliary bli, eg. li porte bli aperta "the door is (being) opened" (ie. someone or something is opening it). The second is expressed with the verb "to be" plus the past participle, eg. li porte es apertat "the door is open" (ie. it is standing open, in an open state).
We now come to another one of my favorite features of the language. There is a regular system of deriving nouns and verbs from each other that runs throughout the language, and seems to apply most of the time.
Nouns from nominal roots, eg. sonje "a dream", can be made into verbs by changing the -e to an -a, thus sonja "(to) dream". By then changing the -a ending of this derived verb into -o, we get the verbal noun sonjo, "(the act of) dreaming". Novial is full of correspondences of this type.
From verbal roots, the situation is somewhat more complex, as there are four different verbal endings:
Sometimes, however, though this process is regularly applied, meanings appear to get somewhat shifted, eg. from destina "(to) destine, fate" we get destino "destiny" and destinatione "destination".
I have compiled lists of the affixes used for wordbuilding in Novial and the prepositions.