Several different areas arise as we consider language changes.
First, words change in meaning—often quickly. Causes include just that something clicks, at least for a while. The slang expression "bad" for good may or may not last. We'll see some changes that did survive. Sometimes social pressures change meanings, as we'll also see. Sometimes it's just hard to tell why a change has happened.
Second, words change in form. Two processes we'll look at involve different issues entirely. The fact that the plural of woman is marked in the first syllable rather than the second is an instance of ease of pronunciation. Another kind of change that can trigger ease-of-pronunciation changes involves contact with other languages. English in its history borrowed Latin words in several periods, first when Britain was a Roman colony (there was Germanic-based Old English spoken before the Anglo-Saxon invasions that follwed the Roman withdrawal). The second great period of Latin word adoption came with the advent of Christianity. The third was during the rebirth of scientific and philosophical interest in the Renaissance and Enlightenment periods. Norse invasions had two kinds of effects: word-adoptions and the radical simplification of case and gender markings. The Norman invasion and years of French influence added many French words. Here's a passage from the prologue to Chaucer's Canterbury Tales (1387-1400) that shows these sources for English words.
OLD ENGLISH || LATIN || OLD FRENCH || OLD NORSE
WHAN that Aprille with his shoures soote
OE hwonne L—>OE OE: scúr OE swoete (OE Ēastermōnað)
The droghte of Marche hath perced to the roote,
OE drúgath F percer N rõt
And bathed every veyne in swich licour,
OE bathian OE æfre F vigne OE swelc F licour
Of which vertu engendred is the flour;
OE hwelc FR vertue Fr engendre F flour
Whan Zephirus eek with his swete breeth (eek or eke, in addition)
OE hwonne OE OE swoete. OE bræth 5
Inspired hath in every holt and heeth
FR inspirer OE æfre OE holt OE hæth
The tendre croppes, and the yonge sonne
Fr tendre OE crop OE geong OE sunna
Hath in the Ram his halfe cours y-ronne,
OE haæalf F cours OE rinnan
And smale fowles maken melodye,
OE smæl OE fugel OE maken F melodie
That slepen al the night with open ye,
OE slāpen OE: æll OE nact OE open OE æge 10
(So priketh hem nature in hir corages:
OE prikath L natura F corage
Than longen folk to goon on pilgrimages, (note the modern "to" and the OE ending on go)
OE thonne OE longian OE folc OE gán F pilgrimage
And palmers for to seken straunge strondes,
F palmere OE sécan F estrange OE strand
To ferne halwes, couthe in sondry londes;
OE feorran OE halig OE cúthe OE syndrige OE land
And specially, from every shires ende
F especia OE fram OE æfre OE sciir OE ende 15
Of Engelond, to Caunterbury they wende,
N thæir OE wendan
The holy blisful martir for to seke, (note, the Greek ancestor word was adopted into Gc)
E halig OE bliths L martir OE sécan
That hem hath holpen, whan that they were seke.
OE hem OE helpan OE hwonne N thæir OE wær OE seoc
Note, by the way, that OE wenden was an intransitive verb, like gán, which replaced wenden completely, demoted it to a transitive verb (He wended his way) and then, to make things even worse, stole its past tense.
Here's another little primer about word changes, from Language Files by the Ohio State Linguistics Department:
He was a happy and sad girl who lived in a town 40 miles from the closest neighbor. His unmarried sister, a wife who was a vegetarian member of the Women's Christian Temperance Union, ate meat and drank liquor three times a day. She was so fond of oatmeal bread made from corn that one night when it was dark and wan out, she starved from overeating. Her brother fed nuts to the deer who lived in the branches of the apple tree from which he picked pears. He was a silly and wise boor, a knave and a villain, of whom everyone approved. He was a lewd fellow whom the general censure held to be a model of chastity.
We can work out a lot of the contradictions ourselves, but the OED can help us out. Words over time adopt a secondary meaning as primary, sometimes seeming to reverse the older meaning. At times a general word narrows, and a specialized word broadens.
One of our nominated words appears here, and we'll explore it and some others in our subpages.