Assuming this question is referring to "late modern English," which is what English speakers and writers use today, the language has several defining characteristics. These are in some cases unique to English, but, broadly speaking, can be said of many other languages as well.
One characteristic is the splintering of the language into distinct but mutually intelligible dialects. This is largely a function of the influence of Great Britain around the world from the seventeenth to the twentieth century. American English is quite different from British English (not that either is a monolithic language—each has different elements of regional slang, pronunciation, and so on) and each is different than the versions of English spoken in Australia, the Caribbean and other locations. This has been been exacerbated by the prevalence of English as a virtual lingua franca in business and diplomacy, and the widespread nature of American pop culture and consumer goods, a consequence of ongoing globalization from the post-World War II era to the present.
Another characteristic is the continuous generation of "neologisms," literally "new words," that have made their way into English. Some languages—notably French, which is curated by a group of scholars known as the Académie française, have been resistant to this process, but English is full of jargon and phrases that reflect both technological change and cultural developments. Some examples of this might include technical terms like e-mail, tweet, and internet, or words that express related concepts—crowdfunding, cyberattacks, or hacking. Some of these might be dismissed as technologically derived slang, but the centrality of information technology in daily life has made them important, and possibly enduring, changes.
Another trend in English is the use of nouns as verbs and vice versa. One source describes this as "verbification" or "nounification" (terms that are themselves neologisms typical of modern English). Some examples of this would be to "model," "message," or "post," on the one hand, or an "ask," a "lift," or a "move" on the other. These modern uses have become very important, to the extent that they represent a distinct characteristic of modern English.
There are, possibly, two answers to your question. It depends on whether you are asking about Early Modern English (Shakespeare's English) or the Late Modern English that is spoken today.
If you are referring to Early Modern English, it began sometime after the Great Vowel Shift (roughly between 1500-1550). Sometimes this form of English (often used by Shakespeare and the original translation of the King James Bible) is called Elizabethan or Early Modern English.
It lacked uniformity. Basically, words could be spelled by sound or however the writer saw fit. Until 1775, when Samuel Johnson published the very first dictionary in Great Britain. In 1828, Noah Webster created his own American dictionary. Thus, differentiating spelling between British Standard English and American Standard English. Early Modern English lasted between 1500-1800. After 1800, more changes in English appeared. This new English would be called Late Modern English.
Some major differences between Early/Late Modern and Middle English are:
- Loss of the T-V distinction (thou, ye)
- The letter thorn (þ) finally fell into disuse. It was replaced by the "th" sound and spelling.
- The letters "i" and "j" became single letters rather than being written together. The same is true for "u" and "v."
- During late modern English, an influx of new words appeared. This happened for two reasons. (1) The Industrial Revolution happened and new words were needed. (2) Also, there was more global interaction, and English began "borrowing" words from other languages.
I hope this helps :)