It would depend on both time and place! This question has been covered quite a few times, including /u/Bayoris who has a great answer which touches upon the aspects that would effect verbal communication – slang, jokes, changes in vowel pronunciations (e.g. the “Great Vowel Shift”), register, etc. – here is their answer . Approaching this question with regard to everyday spoken/verbal language would be out of my depth, so this answer will approach the question with regard to literary/written scenarios. We can start by having a look at some of the earliest written English that survives (Known as Old English), roughly from 600-1100). Caedmon’s Hymn is from the 7th c., and is preserved in the Venerable Bede’s writings (a legend suggests that Caedmon was an illiterate cowherder, and having received the gift of writing from God, composed this hymn (1):

Nu sculon herigean / heofonrices Weard Meotodes meahte / and his modgeþanc weorc Wuldor-Fæder / swa he wundra gehwæs ece Drihten / or onstealde He ærest sceop / ielda bearnum heofon to hrofe / halig Scyppend ða middangeard / moncynnes Weard ece Drihten / æfter teode firum foldan / Frea ælmihtig. (2*)

(Translation: )

[Now must we praise / heaven-kingdom's Guardian,] [the Measurer's might / and his mind-plans,] [the work of the Glory-Father, / when he of wonders of every one,] [eternal Lord, / the beginning established.] [He first created / for men's sons] [heaven as a roof, / holy Creator;] [then middle-earth / mankind's Guardian,] [eternal Lord / afterwards made --] [for men earth, / Master almighty.] (2)

Hwæt the.. - it’s quite unrecognisable! However, if you take a closer look at some words, you might be able to work them out without any dictionaries! Fæder = Father, Heofon = Heaven, ælmihtig = almighty, are a few examples. Some are completely different – swa (sometimes used twice, swa swa, meaning as, or “just as”, or “like”. There are often words that seem like they’re quite unfamiliar to us at first, wis, for example, meaning roughly the manner or way of something – but we often use this today – for example with “clockwise” – the way or manner of a clock. The Old English word stede refers to a “place” (or sometimes a holy site) – we don’t use stede anymore, except with the preposition instead - “in place of”. The digraph æ generally has the sound of the a in the modern English “hat”; ð and þ have a “th” sound, as in modern English “that” or “earth”. You may also come across ƿ, which is pronounced as the modern English “w” as in “win”.

Perhaps one of the biggest differences with Old English compared to Modern English besides its vocabulary, is its use of grammatical cases and gender. Word generally has very little effect on Old English – words take on different spellings (inflections) in order to indicate their grammatical function – either nominative (the subject), accusative (the object), dative (the indirect object), or genitive (possessive). Generally, a single word will have multiple spellings to account for its case, and whether it is plural or a singular, and the pattern by which the spellings change is dependent on the gender – which has resulted in some of the unusual inconsistencies we see with plural words today – neuter nouns in Old English typically made no spelling distinction between singular and plural; an example is sceap (“sheep”), which is the same for both sing. and pl. in the nominative case. In the nominative case, Cyning (“King) has a different plural ending, Cyningas, which occurs for masculine nouns. As a result, the sentence “The King went to the mountains” could be rendered in Old English as “to the mountains King the went” (*þaem beorgum cyning se ferde”) – the grammatical case and gender provide us with the information needed to know who is doing what, as opposed to Modern English’s use of a more strict subject-verb-object word order.