Verb Stems VS Verb Endings
For example, you should now know that the plain past tense of 食べる (たべる // to eat) is 食べた (たべた // ate) but when you want to be a bit more formal, you would use the polite past tense, 食べました (たべました // ate).
-る → -た
-る → -ました
To be conjugation masters, though, we'll need to get our stem changes perfect, too, which is what that cheat sheet in the last lecture was all about:
食べる (たべる) is an ichidan [Group II] verb, so its stem never changes.
Look at what happens with the godan verb 書く (かく // to write), however. Our endings are the same (-た and -ました), but the stems are different:
食べた (たべた // ate)
書いた (かいた // wrote)
食べました (たべました // ate
書きました (かきました // wrote)
In the case of plain past tense for the verb 書く, we're adding い to the verb stem.
In the case of polite past tense, we're adding き to the verb stem.
Learning such stem changes is the real challenge of Japanese verb conjugation, as the endings are typically consistent for all verbs of a particular type.
For example, let's take two godan verbs, 書く (かく // to write) and 買う (かう // to buy). In the following plain past conjugations of different tenses, note how they all end in ～た, yet their stems don't always change in the same way:
|書けた||かけた||was able to write|
|買えた||かえた||was able to buy|
|書かせた||かかせた||made/let (someone) write|
|買わせた||かわせた||made/let (someone) buy|
The majority of the stem changes for each tense are the same for each verb, if perhaps intimidating for a beginner of Japanese.
For past potential/possibility form, the verb-ending kana (in this case ～く for 書く and ～う for 買う) changes from a "-u" sound to an "-e" sound before we add the verb ending (=～た). So we have 書け- (かけ-) and 買え (かえ-).
Something similar happens for causative form, except we're changing the verb-ending kana from a "-u" sound to an "-a" sound before we add the verb ending (that is, the part of the verb ending that is the same for all causative tense conjugations: ～せた). So we have 書か- (かか-) for 書く. For this form, 買う (かう) is actually an irregular verb. The reason is that the "-a" sound version of -う would be -あ, yeah? But we can't put 買あ (かあ) because that just gives us an elongated "-a" sound. Instead, we need to change the "-u" to "-wa," giving us 買わ- (かわ-).
For plain past tense form, however, the verb-stem changes are different. For 書く (かく) we add い, giving us 書い- (かい-), but for 買う (かう) we add a small っ, giving us 買っ- (かっ-)… before adding the verb ending (=～た).
In accordance with the above, you can see that 書ける (かける) and 買える (かえる) are highlighted the same color in the "possibility form" column of our conjugations cheat sheet:
Likewise, 書かせる (かかせる) and 買わせる (かわせる) are highlighted the same color in the "causative form" column:
They are highlighted the same color because their verb stems are changing in the same way.
Since the verb stems are changing in varying ways for plain past tense, the verbs are highlighted different colors in the "Ta form" column:
Thanks to the highlighting in the table, we can also see that verb-stem changes are the same for "command form," "conditional form," and "possibility form:"
And they're the same for "negative plain form," "passive form," and "causative form:"
Finally, depending on the verb ending, they're the same for "te form," "ta form," and the "tara conditional form:"
I've said this already, but I think it bears repeating: You do not have to memorize all of this just yet.
At the moment, we're just trying to recognize patterns among verb stem changes. If we get used to these patterns now, mastering our conjugations will be much less painful in the future.
In the remaining lectures of this section, I'm going to point out the various stem-change patterns we're seeing. There are six sets of them in total.