I translated this sentence into English as "he reads too quickly for me", but Duo says that "quickly" is the "wrong" word and that it should be "fast". I strongly agree with the comments of most of your correspondents below, that Duo is wrong here in not accepting "quickly" as being virtually synonymous with "fast". In the context of this sentence, "quickly" would be the preferable word in English, as Mark 392155, and others below, have quite rightly stated.
For anyone else struggling with me: This must be the most underdeveloped section of the duolingo german course (dative prepositions)..
so much unexplained (i do appreciate what has been explained though, like the contractions), which is odd because prepositions are naturally one of the least straightforward parts of a language; so many acceptable answers have been reported that Duolingo is not accepting after years of reports. Seems like a waste of time to memorize answers that only Duolingo will accept, only to later be confused in a real-life situation where that rigidity is challenged.
A shame no one has answered you. "Too" has a couple of meanings. It is an adverb that adds a vague amount to the adjective or adverb that follow it. For instance: "it is too hot" or "She ran too quickly."
In addition, it means "also." As in "I'm going, too" or "They are happy, too" So you can say "They are too happy, too" but it sounds better if you say "They, too, are too happy." And don't ever start a sentence with "too." You shouldn't say "Too, it means also." Starting a sentence with "too" drives many English speakers crazy.
The other "to" is a preposition. It is also the full infinitive of a verb: "to go" or "to have." You don't use it as a verb, it is just a full expression of the verb before conjugating
I recommend against using "too" twice in a sentence. I think it's better to replace one with "also." For instance, "They are also too happy" sounds better than, "They, too, are too happy." If for no other reason, commas seem to have fallen out of favor, at least in the US & Canada.
Too is an adverb whereas to is a preposition.
Too can mean "also" (I, too, am happy), or it can indicate an extreme (it is too big). We often use too instead of also because it sounds better. for example, the phrase"me also" is correct English, but it sounds odd to the ear, at least to me, so we use too here instead of also.
As stated before, to is a preposition, so it is used to join diffent parts of the sentence together (I am going to his house).