What are the differences between Ichidan and Godan verbs? I’ve seen intransitive and transitive verbs in both categories, and I’ve also heard of Nidan verbs too. What’s the difference?
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Ichidan and godan verbs have different conjugations
Here’s a Tofugu article about it, but basically, ichidan verbs can only end in る, and they usually drop that る when conjugating. Godan verbs can have various -u terminations (including る), and that termination often swaps into another sound of the same column (for example: 好む has the m- termination, so when conjugated it might turn into み, ま, も etc)
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It has to do with the amount of stem forms for conjugation.
Ichidan verbs have one stem form (hence ichidan) - strip off the る, and there you have it. You use that stem form for basically every conjugation.
Godan verbs have five stem forms (hence godan), one for each column of the kana table, ending in each respective vowel.
Nidan verbs are antiquated and probably not something you have to worry about for the foreseeable future, but they have two stem forms, hence nidan.
So for a godan verb like 行く, the stems are
行か - used with for instance the negative form 行かない
行き - used with for instance the polite form 行きます (and many others - this is sort of the “generic noun” form of a verb, and will often be referred to as the “masu stem”, and is more formally known as the renyokei)
行く - this is just the dictionary form
行け - used with for instance the potential form 行ける
行こ - used with for instance the volitional form 行こう
For an ichidan verb like 食べる however, the stem form is just 食べ for all of those:
食べない
食べます
食べる
食べられる
食べよう
Nidan verbs are a little different and their conjugation patterns have been replaced with the ichidan conjugation patterns, but they come in two types: kami (upper) nidan verbs, with a stem ending in -i and a stem ending in -u, and shimo (lower) nidan verbs, with a stem ending in -e and a stem ending in -u. But if you’re going to conjugate those you’ll end up with entirely different forms anyway, since even the conjugations themselves are different in classical Japanese.
Also interesting to know: the kami/shimo distinction also exists for ichidan verbs, and refers to ichidan verbs ending in -iru and -eru respectively (or more accurately, stems ending in -i or -e). As you might expect, these stems are just the respective stems of the original nidan verb. (so 起く for instance, being a kami nidan verb, had its stem form 起き turn into the ichidan verb 起きる, whereas 上ぐ, a shimo nidan verb, had its stem form 上げ turn into the ichidan verb 上げる).
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In jp dictionaries you see that denoted as
食べる（他下一） for example → 他 for transitive 下 for shimo 一 for ichidan.
行く （自五）for example → 自 for intransitive 五 for godan
If you ever wonder what these things in the dictionary are supposed to mean 
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Woah, a buncha knowledge being dropped in this topic. I’m not the OP but ty 
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To throw in some more useless information: along with Nidan in the “obsolete types” bucket, there’s also Yodan. All Yodan verbs became Godan, with the addition of the ～おう conjugation after WWII.
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It should be 食べ ら れる in standard Japanese. It’s true that the so-called ら抜き (‘ra-removed/pulled-out’) forms are fairly common in informal Japanese, but if we’re talking about standard grammar, the ら should be left in there. In essence, the helper verb that gets attached to the stem is a little different for each sort:
食べ+られる for ichidan verbs
行か+れる for godan verbs
@TheGreatRamen: actually, I just posted a series of Tweets on this over the past week. (I’ve finished the verb segment, and have just started the adjective segment.) I don’t cover absolutely everything, but I do cover the five basic stems, which @yamitenshi also covered. I guess I’ll just post the link to the summary Tweet:
https://twitter.com/AnAsianPolyglot/status/1488780528253906945?s=20&t=n3h9NdypIbKcyIi7mRR5DQ
(There are summary tables attached as images with comments. Basically there are also two exceptions that aren’t ichidan or godan – する and くる – and there are two other verb common verbs that I figured I’d mention because they’re slightly different – いく and ある – but not in terms of verb stems. You can see the differences in the past tense and the て-form, mostly.)
If you’d like to see the other Tweets, you can probably just scroll through my profile since I haven’t posted much else lately. Here’s a link to the first Tweet of the series:
https://twitter.com/AnAsianPolyglot/status/1487416215404523522?s=20&t=Jb1zRClyCWwYdRZIjhvKgw
One more thing:
They’re called かみ（上 i.e. up, upper etc.）and しも（下 i.e. down, lower etc.）because in the Japanese order for vowels, I comes before E. (It’s AIUEO.) Indeed, nidan verbs no longer exist in Japanese today, but some people have pointed out that 〜ずる verbs like 信ずる (they’re basically a special class of 〜する verbs) are changing the same way upper nidan verbs did: the U sound is becoming an I sound, especially in the plain form i.e. more and more people act as if 信じる is the default form, whereas technically, 信ずる is the proper/traditional plain form.
While we’re on the subject of verb history, there used to be yodan verbs: those became the godan verbs of today. If I’m not wrong (I haven’t looked into the details yet, so I’m not sure), the reason for the evolution is that the う of the volitional form was traditionally attached to the ‘negative stem’ (which is technically the irrealis form, the mizenkei), and that AU sound turned into a OU sound. For example, I think this is what happened with 行く:
いかむ→いかう→いこう (む is the classical equivalent of the う and よう helper verbs today)
That AU → OU sound shift is actually part of a much wider shift that seems to have occurred across a lot of Japanese, but I don’t know all the details either – just a few other examples – and I think it’s a story for another time. 
EDIT: Oops, I didn’t see that Belthazar had already mentioned yodan verbs… oh well, I hope the details were interesting at the least.
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Whoops! Absolutely right, not sure why I keep forgetting that.
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