The ultimate guide for verb conjugation (Godan and Ichidan verbs)
Since I tend to confuse the verb endings for the different forms of the verbs (non-past polite, negative non-past casual etc.), I decided to make a very short overview here to help me keeping everything apart and finding a structure behind the rules.
Since this is something you might be able to use yourself, I’ll first explain some basic concepts about verb conjugation. If you, however, feel confident that you already know this well enough, feel free to skip it. It’s up to you 
Basic concepts about verb conjugation
When conjugating a verb in Japanese, you’ll first have to find out to which verb group it belongs. There are, as far as I know, only three different types of verbs: Ichidan-Verbs (also る-verbs because in their dictionary form aka plain form, they always end in る, no exceptions), Godan-Verbs (also う-verbs, because in their plain form, they always end in a う-sound, no exceptions), and Irregular Verbs. Unlike in other languages, tho, there are only two irregular verbs, so that shouldn’t be to hard to remember. (If you know any other verbs, please let me know). Since irregular verbs are irregular, I’ll just focus on Ichidan and Godan verbs in this post. If you’re curious: The irregular verbs are 来る (to come) and する (to do).
Note: Every Ichidan verb ends in る, but not every verb ending in る is actually a Ichidan Verb! If you remember, I said that Godan verbs end in an う-hiragana, and that includes sigh る. So, how can you tell whether a verb ending in る is actually an Ichidan verb or a Godan verb that is trying to disguise itself as an Ichidan verb? Well, if the last vowel before る is an a, o or u, then you can 100% tell that the verb is actually a Godan verb. If the last vowel before る is an i or e, you can assume that it’s probably an Ichidan verb.Yes, you heard me right, “probably”. For the verbs ending in -iru and -eru, you’ll just have to learn whether it’s an Ichidan verb or not, so that kinda sucks, but at least there are only two irregular verbs, so at least that’s not a problem 
This is important to know because verbs from different verb classes conjugate differently. So, assuming you now know what Godan and Ichidan verbs are, let’s just get into it!
Endings for conjugations
A verb can be conjugated in eight different forms, these forms being the non-past, the non-past negative, the past and the past negative as well as the polite forms of all of these forms.
To conjugate any verb from these classes, you basically just modify or remove the last kana of the verb and add the appropriate ending. This is true for Ichidan verbs as well as Godan verbs. These endings are:
| Verb form | Ending | Alternate Ending |
|---|---|---|
| Non-Past Casual | ||
| Non-Past Polite | ます | |
| Non-Past Casual Negative | ない | |
| Non-Past Polite Negative | ません | ないです |
| Past Casual | た (Exceptions for Godan Verbs; see below) | |
| Past Polite | ました | |
| Past Casual Negative | なかった | |
| Past Polite Negative | ませんでした | なかったです |
How to conjugate verbs
So, how do you know actually conjugate the verb? Welp, for Ichidan verbs, it’s very simple: You literally just replace the る with the corresponding ending and there you have it! At least that’s easy 
As for Godan verbs, it’s slightly more complicated. Before you attach the ending needed to build the form you need, you have to modify the last kana of the verb. Godan verbs can end in the kana う, く, す, つ, ぬ, む, る,ぐ and ぶ. The modification itself follows these rules:
Non-Past Casual: Ok, here’s nothing to do :3
Non-Past Polite: Here, you have to change the kana to the kana from the same column that end in an い. For example, if your Godan verb ends in a く, you’ll have to change the ending to a き before adding the ending. Note that つ changes to ち and す changes to し.
Non-Past Casual Negative: Here, the last kana will change to the あ-kana within the same column, e.g. る changes to ら and す changes to さ. There is one exception in this column, though, namely う changes to わ instead of あ, so keep that in mind. (Also, つ changes to た, but that’s no irregularity, just wanted to add at.)
Non-Past Polite Negative: If you want to use the ないです, you can literally just make the Non-Past Casual Negative form of the verb and simply add a です at the end. If you, however, use ません, you first have to change the kana from the う one to the い one, just like for the Non-Past Polite form, before adding the ending.
Past Casual: This one’s a bit odd, so let’s talk about it at the end
Past Polite: For this, the kana modification is actually the same as for the polite Non-Past form, hurray!
Past Casual Negative: Here’s the same as with the last one: The kana modification follow the same rule as the kana modification for the negative Non-Past Casual!
Past Polite Negative: Exactly the same as with the Non-Past Polite Negative, just add the right ending and you’re good to go!
Now, to summarise all the kana modifications for Godan verbs in four rules, except for the Past Casual:
1. If you want to use a polite form, you change the う-kana to the い-kana within the same column, because you’re making it POLLite. (Hello POLLfam!)
2. If you want to use a casual negative form, you change the う-kana to the あ-kana within the same column, because you’re negating it. Don’t forget our little exception: う becomes わ!
(Here’s a mnemonic for you: After you read this, you say: “う does NOT become あ??? WAAAAAAAAAAAAAAA, SAVE ME FROM THIS MADNESS!!!”)
3. If you want to make a polite negative form, you’ll use the modification rule for polite conjugations, because being polite, which is a positive thing, is better than being negative.
4. For the polite negative form, you also have the possibility to add a です to the casual negative form, because です makes things polite.
5. The best for the last, if you want to use the casual non-past form, just use the plain form.
The big exception
Now, let’s finally talk about the casual past of Godan verbs:
Here, the rules are very different from the rest, and it’s the only form of Godan verbs in which their ending doesn’t match the general verb endings and the kana modifications. This is why I wanted to save this for the last, as it is the exception to the rule.
First of all, the ending for the past casual for godan verbs is mostly た, there are, however, four exceptions, and these exception occur if the last kana of the verb is either a む, ぬ, ぶ or ぐ. In these cases, the correct ending is だ. Here’s a mnemonic for you to help remembering these four kana:
I usually buy and eat nudles from gucchi, but today, I have to give them to my cow so that she makes MUUUUUUU again.
Then, finally, let’s talk about kana modifications:
- る, う and つ become the small っ, because they all look pretty similar to っ (for る, you have to cut off the top two strokes, which happen to be the only straight strokes as well btw)
- く and ぐ both become い, because if you rearrange the two strokes of the い, you can make a く (and it’s hadukened version as well)
- す becomes し, because everyone likes sushi
- Finally, む, ぬ, and ぶ all become ん. Why this? Because your cow that makes MUUUUU thinks that buying nudles is going to be your nnnnnnnnext problem, considering your financies. (You could, however, sell that cow of your to solve your financial problems
)
That’s it! Unfortunately, I can’t think of anything simpler for the rules of the casual past of Godan verbs.
Summary about verb conjugation for Ichidan and Godan verbs
For everyone who was too lazy to read through everything, I’ll repeat the most important things I said about verb conjugation:
Endings for the different conjugations
| Verb form | Ending | Alternate Ending |
|---|---|---|
| Non-Past Casual | ||
| Non-Past Polite | ます | |
| Non-Past Casual Negative | ない | |
| Non-Past Polite Negative | ません | ないです |
| Past Casual | た (Exceptions for Godan Verbs; see below) | |
| Past Polite | ました | |
| Past Casual Negative | なかった | |
| Past Polite Negative | ませんでした | なかったです |
Ichidan Verbs
Any form = Plain form of the verb without る + correct ending
Exception for Ichidan Verbs: The Non-Past Non-Negative Casual form is just the plain form.
Gondan Verbs
Follow these steps to make any form:
1. Modify the last kana of the Verb in the plain form according to these rules
- a) If you want to use any polite form, change the う-sound kana to the い-sound kana within the same column to make it polite
- b) If you wand to use any negative form, change the う-sound kana to the あ-sound kana within the same column to negate the verb; don’t forget about the exception う—>わ
- c) In case you want to use any polite non-negative form, always follow the rule for the polite kana modification, because being polite, which is a positive thing, is better than being negative.
2. Add the correct ending after you’ve modified the verb. Note that if you want to use a polite negative form, you can also just take the casual negative form of the same tense and add a です to it, because です is the politeness marker.
Exceptions for Godan Verbs:
1. The non-past non-negative form is just the plain form of the verb.
2. The non-negative casual past follow it’s own rules:
-
a) If the ending is む, ぬ, ぶ or ぐ, the ending used to build the past casual non-negative is だ
-
b) The Kana changes are as follows:
- If the last Kana is る, う or つ, it changes to the small っ
- If the last Kana is く or ぐ, it changes to い
- If the last Kana is む, ぬ or ぶ, it changes to ん
- If the last Kana is す, it changes to し
Ok, that’s it. I hope you find this helpful, and if you have any suggestions about adding or changing something, please let me know