Linguistic Rules

1.  A long vowel shortens before:

1)  -nt(-)  
Example:  (3rd person plural ending) vidē-, vident
2)  final -m, -r, -t
Example:  vidē-, videt (3rd person singular)
amor, amōris  (3rd declension nominative singular)
3)  another vowel
Example:  vidē-, videō

2.  vowel-s-vowel > vowel-r-vowel

(“rhotacism” < Greek letter rho)
(esse; stem es-)  (es-am;) eram imperfect
(es-ō) erō future
tempus, (tempus-is) temporis
ōs, (os-is) ōris
(ges- gerō, gerere, gessi, gestus

mutes/stops liquids
voiced unvoiced nasal
(Labium lip)
b p m
(dent- tooth spirant)
d t
n l
(gutter throat)
g c r
semivowels: i = consonant (“y” as in “you”):  at the beginning of a word when followed by a vowel OR in the middle of a word between two vowels
e.g.: iuvō; iam; maius
u = consonant (“w” sound): at the beginning of a word when followed by a vowel or in the middle of a word between two vowels or before another “u.”
e.g.:uōs; cognōuī; seruus

3.  g + s = x

(x < -c/-g + s)
3rd declension nominative singulars:
  (rēg-s) rēx, rēgis
perfect active verb stems:  regō, regere, (rēg-si) rexi
vīvō (root vīg-), (vīg-sī) vīxī

4.  c + s = x

  (x < -c/-g + s)
3rd declension nominative singulars:
(pāc-s) pax, pācis
perfect active verb stems:
  dūcō, dūcere, (dūc-sī) dūxī

5.  d or t (dental) + s = long vowel/syllable -s or -ss (with loss of d/t)

3rd declension nominative singulars: 
laus (laud-s), laudis
ars (art-s), artis
aetās (aetāt-s), aetātis
verbs:  ascendō, ascendere, ascendī, ascensus (ascend-sus)
mittō, mittere, mīsī (mitt-sī), missus (mitt-sus)
(pot-sum) possum, potēs, etc.
videō, videre, vīdī, vīsus (vīd-sus)

6.  b (voiced) + s (unvoiced) = -ps- (both unvoiced)

scrībō, scrībere, (scrīb-sī) scrīpsī

7.  g (voiced) + t (unvoiced) = -ct- (both unvoiced)

regō, regere, rexī, (reg-tus) rectus

8.  b (voiced) + t (unvoiced) = -pt- (both unvoiced)

scrībō, scrībere, scrīpsī, (scrīb-tus) scrīptus

9.  “m” before “c” or “d” < "n"

num-c < hunc
quem-dam < quendam

10.  ā/ab:

ab: always before vowel or h
sometimes before consonants, especially l, n, r,(liquids); s
rarely before c, consonantal i, d, t
almost never before p, b, f, v,
ā: regularly before stops, especially labials
N.B. ā/ab does not assimilate in compound verbs; it there is assimilation, the prefix is ad-, not ab-.

Pattern of Vowels

Front (Central) Back
i u Close
e o (Mid)
a Open
            (See also Allen Vox Latina)

11.  vowel opens or rounds before “-r(-)”

dīcit, (dīc-i-re) dīcere, (dīc-i-ris) dīceris  (i < e)
tempus, (tempuris by #2) temporis  (u < o)
thus: syllabic “r” becomes -er- (not -ir):
2nd declension  nouns with bases in -r:
nominative singular: ager  (base agr-)

12.  weakest vowel -i (added when vowel is needed and there is not one there already)

adverbs:  -(i)ter
3rd conjugation:  (dīc-s) dīcis
(dīc-re < dīcire < dīcere by #11)
Except:  syllabic -n/-m becomes –un/um:
e.g. sum (< s-m);
cf. 3rd conjugation (dīcunt instead of dīcint (< dīc-nt)
See also #11 above (i

13.  Vowel weakening:  when words are compounded, the medial vowel often weakens (closes):

a < i:  in short syllable (before a single consonant or -ng)
faciō, per-ficiō
a < e:  in long syllable (before two or more consonants)
capiō, captus, incipio, inceptus
e < i:  in short syllable (before single consonant)
teneō, con-tineō
ae < i: 
quaerō, re-quī
au < ū:
claudō, in-clū

14. sum, esse, fuī, futūrus to be (stem: es-/s-)

s – m
> sum s – mus
> sumus
es – s > es es – tis > estis
es – t > est s – nt
> sunt
Note: syllabic m/n comes out as um/un in Latin
° ° For the imperfect and future (er-), see Linguistic Rule 2.