I would like to clarify that Chaucer is Late Middle English, and English standards were in large part based on his writing, so his writing is generally understandable for most English readers. Early & Middle Middle English, though, not so much... Here's a poem written at the start of the Early Middle English period by a very conservative writer, showing a transition betwen Old English and Early Middle English, called Durham (written in the early 12th century):

Is ðeos burch breome geond Breotenrice, steppa gestaðolad, stanas ymbutan wundrum gewæxen. Weor ymbeornad, ea yðum stronge, and ðer inne wunað feola fisca kyn on floda gemonge. And ðær gewexen is wudafæstern micel; wuniad in ðem wycum wilda deor monige, in deope dalum deora ungerim. Is in ðere byri eac bearnum gecyðed ðe arfesta eadig Cudberch and ðes clene cyninges heafud, Osuualdes, Engle leo, and Aidan biscop, Eadberch and Eadfrið, æðele geferes. Is ðer inne midd heom Æðelwold biscop and breoma bocera Beda, and Boisil abbot, ðe clene Cudberte on gecheðe lerde lustum, and he his lara wel genom. Eardiæð æt ðem eadige in in ðem minstre unarimeda reliquia, ðær monia wundrum gewurðað, ðes ðe writ seggeð, midd ðene drihnes wer domes bideð.

Here's part of the opening of Layamon's Brut, from around 1200 CE (notice the simplified grammar / less inflection, and more familiar spelling):

An preost wes on leoden; Laȝamon wes ihoten. he wes Leouenaðes sone; liðe him beo Drihten. He wonede at Ernleȝe; at æðelen are chirechen. vppen Seuarne staþe; sel þar him þuhte. on-fest Radestone; þer he bock radde. Hit com him on mode; & on his mern þonke. þet he wolde of Engle; þa æðelæn tellen. wat heo ihoten weoren; & wonene heo comen. þa Englene londe; ærest ahten. æfter þan flode; þe from Drihtene com. þe al her a-quelde; quic þat he funde. buten Noe.& Sem; Iaphet & Cham. & heore four wiues; þe mid heom weren on archen. Laȝamon gon liðen; wide ȝond þas leode. & bi-won þa æðela boc; þa he to bisne nom. He nom þa Englisca boc; þa makede Seint Beda. An-oþer he nom on Latin; þe makede Seinte Albin. & þe feire Austin; pe fulluht broute hider in. Boc he nom þe þridde; leide þer amidden. þa makede a Frenchis clerc; Wace wes ihoten; þe wel couþe writen. & he hoe ȝef þare æðelen; Ælienor þe wes Henries quene; þes heȝes kinges. Laȝamon leide þeos boc; & þa leaf wende. he heom leofliche bi-heold. liþe him beo Drihten. Feþeren he nom mid fingren; & fiede on boc-felle. & þa soþere word; sette to-gadere. & þa þre boc; þrumde to are. Nu bidde[ð] Laȝamon alcne æðele mon; for þene almiten Godd. þet þeos boc rede; & leornia þeos runan. þat he þeos soðfeste word; segge to-sumne.

Here's part of The Owl and the Nightingale, 12th or 13th century:

ICH was in one sumere dale, in one suþe diȝele hale, iherde ich holde grete tale an hule and one niȝtingale. Þat plait was stif & starc & strong, sum wile softe & lud among; an aiþer aȝen oþer sval, & let þat [vue]le mod ut al. & eiþer seide of oþeres custe þat alre-worste þat hi wuste: & hure & hure of oþere[s] songe hi holde plaiding suþe stronge. Þe niȝtingale bigon þe speche, in one hurne of one breche, & sat up one vaire boȝe, - þar were abute blosme inoȝe,- in ore waste þicke hegge imeind mid spire & grene segge. Ho was þe gladur uor þe rise, & song auele cunne wise: [b]et þuȝte þe dreim þat he were of harpe & pipe þan he nere: bet þuȝte þat he were ishote of harpe & pipe þan of þrote. [Þ]o stod on old stoc þar biside, þar þo vle song hire tide, & was mid iui al bigrowe; hit was þare hule earding-stowe. [Þ]e niȝtingale hi iseȝ, & hi bihold & ouerseȝ, & þuȝte wel [vu]l of þare hule, for me hi halt lodlich & fule. "Vnwiȝt," ho sede, "awei þu flo! me is þe w[u]rs þat ich þe so. [folio 233r.2] Iwis for þine [vu]le lete, wel [oft ich] mine song forlete;

Some text from Ormulum by Orm, who attempted to reform & regularize English spelling, 13th century:

Forrþi þatt Sannt Johaness word Sprang wide & side o lande, Forr þatt he wass u¯¯tnumenn mann I laress & i dedess, & forr þatt he bigann himm sellf, Þær þær he wass i wesste, To fullhtnenn baldeliȝ þe follc Þatt nass næfrær bigunnenn; Hiss word sprang inntill Ȝerrsalæm & inntill all þatt ende, & all þatt follc toc niþ wiþþ himm Þatt læredd wass o boke, Off þatt he toc swa baldeliȝ To spellenn & to fullhtnenn,Page  2, vol. 2 Alls iff he wære Drihhtin Crist Þatt tanne cumenn wære. & teȝȝ þa tokenn sanderrmenn Off preostess & off dæcness & senndenn u¯¯t off Ȝerrsalæm Till himm inntill þe wesste, Forr þatt teȝȝ sholldenn fraȝȝnenn himm, & asskenn whatt he wære. & swa þeȝȝ didenn, & he þeȝȝm Ȝaff sware onnȝæn, & seȝȝde; Namm I nohht Godess Sune, Crist, Þatt cumenn amm to manne. & teȝȝ onnȝæn till himm; arrt tu Helyas efft o life? & he till hemm; naȝȝ, namm I nohht Helyas efft o life. & ȝe¯¯ teȝȝ seȝȝdenn þuss till himm; Seȝȝ uss, arrt tu profete? & he þa seȝȝde þuss till hemm; Naȝȝ, namm I nohht profete.

Taken from Robert Mannyng's Handlyng Synne, he was from Northern England and lived near Cambridge, 14th century (this one is pretty readable for English speakers, other than the words no longer used in Modern English):

Þe fyrst poynt of þy shryfte oweþ to be, 'with þy gode wyl and hertë fre, whan þou art yn þy lyfë best, Awey þou shalt þy synnë kest;' For þan wurschepest þou God aboue, And forsakest þy synne for hys loue. Dauyd seyþ, yn wurde to fulfyl, God shrofe hym with hys gode wyl; Abyde nat tyl þou most nedly, For þan hyt ys with fors and maystry; Þou shryuest þe þan for drede, Nat for loue, but more for nede; Þan semeþ hyt þat hyt shulde be Þat þy synne forsakeþ þe. Swyche shryftë made Acor þe þefe: whan he shulde go to pynë grefe, He shrof hys synne, how he hadde stole Þre þyngës þat he hade forhole, A mantle of skarlet, a rolle of golde, And syluer, he hydde vndyr molde; Yn þe byble men mow hyt se, Yn þe story of Iosue.

Chaucher's The Book of the Duchess, Chaucer is important for reviving English as a language used for art (ever since the Norman conquests, English usage was stigmatized and the prestige languages were French and Latin, so there's very little Middle English literature up until Chaucer), he introduced a lot of now commonly used vocabulary and was the example for English poetry for a long time.

William Caxton's Blanchardine, a less-than-ideal translator who was the first person to introduce the printing press into England and the first English retailer of printed books, late 15th century, around when Late Middle English started to transition into Early Modern English (and also right around the time period that English-speakers, including literate people, started colonizing the Americas):

The first Chapter entreateth of the byrth of Blanchardine, his nurssing and his bringing vp. AMongst many antient Chronicles importing the haughtie exploites of sundry nations, Lords and Princes: this story of the valorous Blanchardine deserueth greatest commendation of true and perfect magnanimitie. At the time when a generall peace concluded throughout the most part of Christēdome, when gentlemen and Noble Péeres made their returne from armes and applyed them selues to domesticall and cuntrie pastimes: There reigned a King in the Realme of Frize redoubted for manhood and prowes, abounding in goodes and possessions, reuerenced and beloued both of his subiect•… and equalls, and most happie in all his attempts [sauing in ye want of issue] to succéed him in his king dōe, wherof, bothe he and the Queene his wife were most pensiue and discontented: and by mutuall complaints greatly bewailed this misfortune. But the Queene desirous to frustrate the scandal that might aryse by reason of her ba•…ēnes: day by day, (in moste deuout and humble manner) solicited the Gods to send her a sonne, whose dayly and deuout prayers penetrating the heauens, by the permision and fauour of the Gods she conceiued and bare a sonne, whome at the christning they named Blanchardine. This vnexpected ioy bred such a content∣ment to the King and Quéene, and a comfort to all the Realm•…, that after humble thankes giuen to the Gods: feasts, banquets and all triumphes were proclaimed throughout the Realme of Friz And least the Childe (should by absence from the Pa∣rents through negligence of the Nursse) miscarie: the Queene her self vndertooke his nurssing and bringing vp. The Childe grew in beautie, proportion, wit and manners, beyond the expectation of all men. And when he was arriued to the age of discretion, he was committed to the tuition of a graue and learned Tutor to be instructed in Philosophie, by whose in∣dustrie and painful labour he soone attained to perfection: his exercises and disportes at idle times were hawking, hunting, and playing at Chesse, and in such pastimes he spent some part of his youth til his maturitie and riper age, made him desirous to follow armes and feates of chiualrie, as heerafter you shall heer.

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