1. When was it?

1066 – 16th century

The height of Middle English literature is generally seen as the 14th-15th century.

2. Who was writing?

3. History

The Norman Conquest of 1066 saw much of modern-day England fall to William the Conqueror, a duke in Normandy, France. With William’s arrival came a variant of language called ‘Anglo-Norman’. French vocabulary, alongside French names now commonly used in English (such as William and Robert), began to take over much of England. Mixed with Old English and Latin, this new language has become known as ‘Middle English’. 
That the rulers of the country were speaking this new English helped push its importance, especially to educated people of standing. The growth of London as a cultural centre did likewise. The invention of the printing press (c. 1440) also helped cement this new form of English as a common tongue.
Yet language in England remained highly fragmented by geography and history. Scotland, where ‘makars’ wrote about about royalty and courts, was also using its own variants. As a result, the term ‘Middle English’ does contain a level of flexibility and overlap (both chronological and geographical).

4. Traits

Middle English texts are not particularly widespread, so the sample size for analysing ‘common traits’ is not particularly large. However, Christianity, travel, and adventure are all heavily utilised themes.
Modern readers find Middle English hard to understand, but much easier than Old English. New letters, still used today, began to appear during Middle English and therefore create something semi-recognisable to the modern eye.
The connection to France – as opposed to being insular or being influenced by Scandinavia – introduced many French words during the Middle Ages. Many of the French words that arrived are still used today, albeit evolutions in spellings can be a problem to present-day readers.

5. Timeline

6. Examples

Sir Gawain and the Green Knight 
Author unknown

Excerpt from Part 2, Lines 670-690:

He sperred þe sted with þe spurez and sprong on his way,
So stif þat þe ston-fyr stroke out þerafter.
Al þat seȝ þat semly syked in hert,
And sayde soþly al same segges til oþer,
Carande for þat comly: ‘Bi Kryst, hit is scaþe
Þat þou, leude, schal be lost, þat art of lyf noble!
To fynde hys fere vpon folde, in fayth, is not eþe.
Warloker to haf wroȝt had more wyt bene,
And haf dyȝt ȝonder dere a duk to haue worþed;
A lowande leder of ledez in londe hym wel semez,
And so had better haf ben þen britned to noȝt,
Hadet wyth an aluisch mon, for angardez pryde.
Who knew euer any kyng such counsel to take
As knyȝtez in cauelaciounz on Crystmasse gomnez!’
Wel much watz þe warme water þat waltered of yȝen,
When þat semly syre soȝt fro þo wonez
þad daye.
He made non abode,
Bot wyȝtly went hys way;
Mony wylsum way he rode,
Þe bok as I herde say.

*Trans:
He struck his steed with the spurs and sped on his way
So fast that the flint-fire flashed from the stones.
When they saw him set forth they were sore aggrieved,
And all sighed softly, and said to each other,
Fearing for their fellow, “Ill fortune it is
That you, man, must be marred, that most are worthy!
His equal on this earth can hardly be found;
To have dealt more discreetly had done less harm,
And have dubbed him a duke, with all due honor.
A great leader of lords he was like to become,
And better so to have been than battered to bits,
Beheaded by an elf-man, for empty pride!
Who would credit that a king could be counselled so,
And caught in a cavil in a Christmas game?”
Many were the warm tears they wept from their eyes
When goodly Sir Gawain was gone from the court
That day.
No longer he abode,
But speedily went his way
Over many a wandering road,
As I heard my author say.

The Canterbury Tales 
by Geoffrey Chaucer

Excerpt from The General Prologue, Lines 19-34:

Bifil that in that seson, on a day,
In Southwerk at the Tabard as I lay
Redy to wenden on my pilgrymage
To Caunterbury with ful devout corage,
At nyght was come into that hostelrye
Wel nyne and twenty in a compaignye
Of sondry folk, by aventure yfalle
In felaweshipe, and pilgrimes were they alle,
That toward Caunterbury wolden ryde.
The chambres and the stables weren wyde,
And wel we weren esed atte beste;
And shortly, whan the sonne was to reste,
So hadde I spoken with hem everichon
That I was of hir felaweshipe anon,
And made forward erly for to ryse
To take our wey, ther as I yow devyse.

*Trans:
It happened that, in that season, on a day
In Southwark, at the Tabard, as I lay
Ready to go on pilgrimage and start
To Canterbury, full devout at heart,
There came at nightfall to that hostelry
Some nine and twenty in a company
Of sundry persons who had chanced to fall
In fellowship, and pilgrims were they all
That toward Canterbury town would ride.
The rooms and stables spacious were and wide,
And well we there were eased, and of the best.
And briefly, when the sun had gone to rest,
So had I spoken with them, every one,
That I was of their fellowship anon,
And made agreement that we’d early rise
To take the road, as I will to you apprise.