Translation:It is neither fast nor slow.
Honestly, it'd be challenging to find any course book that begins and enforces most of the common Kanji immediately upon introducing words.
The challenge there is that you'd be working with the Kanji before mastering the spoken words or remembering which sounds are longer or shorter.
Certainly, duo would benefit from including a toggle to switch between kana and kanji. However, like all one size fits all programs, it doesn't support such complexity and you'd benefit more from assuming that it won't do so before you master all of its Japanese courses.
So, like all learning tools, you'll have to work with different resources. This will especially be true on aspects like kanji, writing, grammar, listening & speaking practice, and extended vocabulary separately from duolingo.
I would like to add that learning vocab words with kanji for them at the same time is ABSOLUTE HELL. Besides Duolingo, I also use Memrise, and there are a lot of courses that teach you vocab words with the kanji, so if I take courses like that, not only do I have to focus on the kanji of this word that I barely even know yet, but also the meaning of the word as well. It's much better to learn the vocabulary words first, AND THEN the kanji for them. Doing them at the same time just destroys my brain.
Lingodeer let's you choose between full kanji (well, at least the most common), hiragana, romaji or any combination of those, which is really nice.
I've been using it at the same time as Duo (as well as the Kanjidamage flashcards on Anki, which doesn't really match up with the others) and I've found it to be really beneficial.
The nihongoshark system that uses James Heisig's Remembering_the_Kanji focuses on learning the entire set of 2200 kanji before learning vocabulary. The theory is that for adults it is easier to learn the kanji ordered by characters with the same elements than one at a time as you learn vocabulary.
That kanji is not normally used in this context, in fact I see it used for talking about car speed, gear speed, that kind of thing. It is an alternative, though, potentially.
Actually, I thought about it more decided to look a bit more deeply, and basically came away with the conclusion that when you want the sense of "early" it's best to use 早 and when you want the sense of fast 速, but these concepts don't map one-for-one onto Japanese, so it's not always as simple as that.
For example, getting away quickly would the second, right? Not really, no, the first because it is more like "as soon as possible" (time) rather than "at great speed" (motion).
Anyway, pretty academic discussion at this point for most learners. :)
Which context? The train is neither early nor late. (It is on time.) The train is neither fast nor slow. We don't even know what "it" is here. All I can say is that Duo does not accept "I am/She is/They are neither early nor late" etc., so it might not expect a person to be neither early nor late, but maybe it should accept it since there is no context given WHO or WHAT is neither early/fast nor late/slow.
Osoi is an "i" adjective meaning slow or late. These words are not adjectives like in English but change their form a bit like verbs. Osokunai is the negative familiar form like adding "not" in English. Add desu to make it polite. Add katta or nakatta after removing the i ending to form the past tense of the adjective in the affirmative or negative.
も is used to link different elements that belong to the same part of speech. You get the meaning both/either or neither/nor depending on tense. も appears after each element in a list - an exhaustive list. と can only be used with nouns (and noun phrases) so it can't be used here.
Theoretically you could say this but two things I'd like to mention:
1) It's unnecessarily complicated and will sound very weird.
2) I understand what you wanted to say but you made a grammatical mistake by not properly conjugating 速くない and adding an unnecessary も. What it should look like (if you want to say it like this):
I'm still confused about the difference between (e.g.) はやい vs. はやくand おそい vs. おそく. If the word ending in -ku are adverbs, why do they still modify the noun, represented by the implicit pronoun "it"? I thought adverbs can only modify verbs, adjectives, or other adverbs. But this isn't the case in this and other, similar sentences Duo has given us.
Any light on this situation would be appreciated. Maybe it's just an artifact of my trying to use English grammar on 日本語 sentences.
I'm not entirely sure, but I think in this sentence it's not an adverb, but the negation of the adjective. In Japanese you can directly negate adjectives or set them into the past: はやい (fast) - はやくない (not fast) - はやかった (was fast) - はやくなかった (was not fast) (This conjugation is for i-adjectives only, btw.)
I can't tell you though why the も particle is in between this constellation.
I don't know too much on this subject, but はやい is an i-adjective. i-adjectives are modified into -くない when negated. For example:
外は暑くないです。(そとはあつくないです。) - It is not hot outside.
The も's in the sentence are also where the 'nether' and 'nor' stem from. This is one of the other ways も is utilized. (From the usual way we see it being used as: 'also')
Hopefully I explained that well enough, and not incorrectly. I'm still learning it all as well.