He can't say any word or understand the world, but when he meets Frankenstein he knows a lot.
It states in the book itself that the monster hid and watched a family interact, and learned words and things such as advanced literature that way. There are some theories I've heard about how the monster could learn so quickly. The monster may actually have been more intelligent or a quicker learner than the average human. Perhaps the monster was like a young child, and children are much more capable of fluently learning languages than adults are.
By stages. In Chapter 12 he learns to speak:
"By degrees I made a discovery of still greater moment. I found that these people possessed a method of communicating their experience and feelings to one another by articulate sounds. I perceived that the words they spoke sometimes produced pleasure or pain, smiles or sadness, in the minds and countenances of the hearers. This was indeed a godlike science, and I ardently desired to become acquainted with it. ...
In Chapter 13 he learns to read:
"While I improved in speech, I also learned the science of letters as it was taught to the stranger, and this opened before me a wide field for wonder and delight.
"The book from which Felix instructed Safie was Volney's Ruins of Empires. I should not have understood the purport of this book had not Felix, in reading it, given very minute explanations. He had chosen this work, he said, because the declamatory style was framed in imitation of the Eastern authors. Through this work I obtained a cursory knowledge of history and a view of the several empires at present existing in the world; it gave me an insight into the manners, governments, and religions of the different nations of the earth. I heard of the slothful Asiatics, of the stupendous genius and mental activity of the Grecians, of the wars and wonderful virtue of the early Romans—of their subsequent degenerating—of the decline of that mighty empire, of chivalry, Christianity, and kings. I heard of the discovery of the American hemisphere and wept with Safie over the hapless fate of its original inhabitants.
And in Chapter 15 he graduates to highbrow literature: Milton, Plutarch, Goethe:
Fortunately the books were written in the language, the elements of which I had acquired at the cottage; they consisted of Paradise Lost, a volume of Plutarch's Lives, and the Sorrows of Werter. The possession of these treasures gave me extreme delight; I now continually studied and exercised my mind upon these histories, whilst my friends were employed in their ordinary occupations.
"I can hardly describe to you the effect of these books. They produced in me an infinity of new images and feelings, that sometimes raised me to ecstasy, but more frequently sunk me into the lowest dejection. In the Sorrows of Werter, besides the interest of its simple and affecting story, so many opinions are canvassed and so many lights thrown upon what had hitherto been to me obscure subjects that I found in it a never-ending source of speculation and astonishment. The gentle and domestic manners it described, combined with lofty sentiments and feelings, which had for their object something out of self, accorded well with my experience among my protectors and with the wants which were forever alive in my own bosom. But I thought Werter himself a more divine being than I had ever beheld or imagined; his character contained no pretension, but it sank deep. The disquisitions upon death and suicide were calculated to fill me with wonder. I did not pretend to enter into the merits of the case, yet I inclined towards the opinions of the hero, whose extinction I wept, without precisely understanding it.