The language of the Stewards is a variation of Sindarin,
the Noble Tongue of the elves. It has rather complicated
grammar so I will only go over some of the major points.
For a complete analysis visit: The Sindarin Grammar Site
and get ready to bang your head against the wall :)

Making Something Plural
Plurals are formed by changing the vowels in the word.
There are two rules for each vowel, one as the first
syllable and one for when it is the last. For example
Adan (man) becomes Edain when pluralized.
I have the original first, followed by when it is
the first, and then when it is the last.

a > e > ai
e > e > i
i > i > i
o > e > y
u > y > y
au > oe > oe

Things To Remember

There is no "to be" in Sindarin
y is pronnounced i (this) in Gondor
-on (augment) -et(diminut) ú-(ill) pen- & al-(negative)

Adjectives
The adjective usually goes after the noun: "annon edhellen" (Elvish gate)

Verbs
All examples are using "ped-" (To Speak)

pidon 'I am speaking'
pidam 'we are speaking'
pida 'he is speaking'
pidar 'they are speaking'
pennin 'I spoke'
pennim 'we spoke'
pent 'he spoke'
pennir 'they spoke'
pedithon 'I will be speaking'
peditham 'we will be speaking'
peditho 'he will be speaking'
pedithor 'they will be speaking'

The adverbs
Here are some commonly used adverbs:
about(o) above(or) afar(palan) after (ab) again(ad) ago(io) always(ui) at(na) behind(adel) enough(farn) fast(lim) here(si) how(manen) in(ned) last(vedui) long(anann) out(ed) not(ú) now(si) there(ennas) through(ter) to(an) today(sir) towards(na) well(mae) when(ir) where(mas) with(na) without(pen)

Conjunctions  
dan 'but', ar 'and', sa 'that', egor 'or'

Words Boromir Might Have Used
auth "war", verb dagro- "to battle, make war", verb maetha- "to fight", verb degi- "to slay" (past tense perhaps *danc), dangen "slain" (as noun), maethor "warrior", herth "troop" (also used for "household"), gweth "troop of ablebodied men, host, regiment", coth "enemy, enmity", dagor "battle" (but a fight between two or a few is called a maeth), hûl "cry of encouragement in battle", megil or magol "sword", lang "cutlass, sword", crist "cleaver, sword", hathel "broadsword-blade", sigil "dagger, knife", grond "club", cú and peng "bow", ech "spear", naith or aith "spearpoint", thôl "helmet".

dôr (dor) "land", gardh "realm, a more or less bounded or defined place, a region" (so in WJ:402; the Etymologies has ardh), sad "place, spot", rain "border", parth "field", pel "fenced field" (pl. peli), orod "mountain" (pl. ered or eryd), till and rass "horn", amon "hill" (pl. emyn), tunn "hill, mound", dol or dôl "hill, head", penn "declivity", ambenn "uphill", dadbenn "downhill", talad "an incline, slope", cîl "cleft", ris or ress "ravine", iau "ravine, cleft, gulf" (this word also means "corn", see above), talf "flat field", nan "valley" (but nann "wide grassland"), tum "deep valley, under or among hills", athrad "ford, crossing", eryn "wood", taur "huge forest", men "road", ael "pool, lake" (pl. aelin), lîn "pool", eithel "spring, issue of water", habad "shore", sîr "river" (in some names also duin: Anduin, Baranduin, Esgalduin), hûb or hobas "haven" (also cirban), gaear (or gaer) "sea", toll "island". Directions: Forod "North", Harad "South", Annûn "West", Amrûn "East". For "East" and "West", the words rhûn and dûn are also used (cf. Dúnedain "Westmen").

aran "king", rîs "queen" (cf. also rien, rîn "crowned lady"), cunn "prince", hîr "lord, master" (another word for "lord" is brannon), hiril "lady" (also brennil, the fem. counterpart of masc. brannon just like hiril corresponds to masc. hîr), arphen "a noble", ithron (or curunir) "wizard", condir "mayor", rochben "knight"