First, we have to be clear that we are talking about the language, not the writing system. The English writing system is indeed complicated, but not because the language itself is complicated. The writing system is complicated because it was first developed for Old English or Anglo-Saxon back before the year 1000 CE. It was developed by Christian missionaries who mostly used the Latin alphabet, and it was not bad. The vowels a e i o u had their Latin qualities (pretty much modern Spanish). A word like Moon (for the Moon) was pronounced like modern English moan. Hus (house) rhymed with modern loose. All the written consonants in cniht (knight) were pronounced. These sounds changed, and some disappeared, over the years, but how we write them did not change so much. We can safely that English spelling is a nightmare. George Bernard Shaw captured this with his humorous observation that we could spell fish as ghoti (gh from enough, o from women, and ti from nation).
If anything, English grammar has simplified over the years. In Old English, nouns had cases like modern Russian and German, so the word for 'person,' folc, varied depending on whether it was singular or plural as well as whether it was being used as a subject, object, possessor, or object of a preposition: subjects and objects singular and plural folc; possessors folces, folca; prepositionals folce, folcum. This meant that word order was somewhat less important in Old English. In these example, the words for king and bishop do not change form, but the definite article does (ϸ is a rune letter that represented the 'th' sounds):
Se cyning meteϸ ϸone biscop. 'the king met the bishop'
ϸone cyning meteϸ se biscop. 'the bishop met the king'
While English has lost some of its complexity, the one thing that probably troubles adult learners most is the auxiliary verb (AUX) DO that makes an appearance in question-formation. Note the following:
Statement: Cats eat mice.
Question: Do cats eat mice?Where did that do come from? English is the only language I know about that forms questions in this way (though there may be others). The answer lies in realizing that the basic rule for question-formation in English is "Move AUX" which means the Auxiliary verb is moved to the front of the sentence. In sentences that already have an auxiliary verb, things are pretty simple. Here the AUX will is moved to form the question:
Statement: The cats will eat the mice.
Question: Will the cats eat the mice?There appears to be no AUX in the statement. Or isn't there? It turns out that all English sentences have a place for an AUX, but the place can be unoccupied. But to follow the "Move AUX" rule and form a question, if the AUX position is empty, we have to create a "dummy AUX" and then move it:
Statement: Cats ___ eat mice. → Cats DO eat mice.
Question: DO cats eat mice?
Statement: Our cat eats mice.
Prep for question: Our cat DO eats mice. → Our cat DOES eat mice.
Question: Does our cat eat mice?Note that the -s on eat moves to AUX before AUX is moved to form the question. There are explanations for why this happens, but they will take us too deep into the weeds of English syntax. For now, let's just say that while English is not as complicated as many people think, it does have its quirks.