Basic politeness and ます form

Let's now talk about politeness. In Japanese, the polite tone of the language presents itself as a specific grammatical feature. This means that when you are speaking politely, the way you build sentences, conjugations, and even the words you choose will differ from when you’re speaking casually. There are various degrees of politeness and we will only cover the basic stuff here. You can look at the intermission if you want to know more, but it is entirely optional.

Basic polite verbs are constructed with the auxiliary verb ます attached to a stem (which is often called the “ます-stem”). For ichidan verbs, ます attaches to the part of the verb without る. For godan verbs, change the u to an i, then attach ます.

In lesson 9, we mentioned that the past tense of godan verbs comes from the "i" stem. While for some verbs (死ぬ -> 死んだ) this stem shows an exception in past form, this does not happen with ます.

Here's a comparison between the past tense and ます form of each type of godan verbs. Don't memorize this.

Since ます itself is an auxiliary verb that ends like any other verb, it can also be conjugated further to provide tense, negativity, etc:

ません doesn't have a past tense of its own. To use the past tense with ません, you attach でした, the past tense of です.

In modern Japanese, there is another way to make the polite negative past tense of verbs, and that is to just add です after the non-polite negative past tense:

死ななかった		->	 死ななかったです  
Did not die (casual)	->	 Did not die (polite)

This usage of です instead of ます can be seen as slightly less polite but has become more and more common to the point where it’s completely normal to use and the distinction between the two is so incredibly nuanced it is impossible to explain. You can just consider them to be the same.

The same applies for ないです vs ません as in 食べないです vs 食べません