The answer is actually rather complicated and has a lot to do with how Japan works. Basically, masu is a respectful form a verb. Yes. You read that right.
It's like, in English, we have "to eat", present, "ate", past. In Japanese, we have taberu 食べる, presente, tabeta 食べた, past, tabemasu 食べます, present polite, and tabemashita 食べました, past polite.
So masu is present polite and mashita is the past tense of masu. It's important to note that these two forms, masu and mashita, are literally the same fucking thing as the normal, impolite present and past forms. They behave the same way both grammatically and syntactically and have the same meaning.
- masu ます
- mashita ました
- masen ません
Polite present negative.
- masen deshita ませんでした
Polite past negative.
The only difference between using the masu form and not using masu form is the nuance. The masu is polite, the non-masu is not polite. That's it. That's all there's to it. Just pretend every time a character says something ending with masu a monocle and a top hat pops up in their design and you'll be alright.
This is kind of a big deal, though. All Japanese verbs have a masu form. All of them. Any verb can end with masu if the speaker wants to, so it's important to remember it doesn't actually mean anything. See examples:
- iku. ikimasu. 行く。行きます。
- korosu. koroshimasu. 殺す。殺します。
- koroshita. koroshimashita.. 殺した。殺しました。
- Motsu. Mochimasu. 持つ。持ちます。
- tabesaseru. tabesasemasu. 食べさせる。食べさせます。
Force someone to eat. Force someone to eat.
As you can see, I have literally repeated the translation in English because there is really no change in meaning here, there is only a change in nuance. Translators probably struggle with this one because conveying that sort of nuance in fansubs or scanlations ought to be very difficult.
In some cases, changing a word from non-polite to polite forces the translation to change in style. See:
- teki ga kuru! 敵が来る
The enemy is coming!
- wakatta! 分かった！
- wakarimashita! 分かりました！
Very well, I understand the information passed on perfectly and am very grateful. Cheers. May the force be with you.
Yes... it's very difficult to convey change in nuance. Sometimes characters speak like that because they are from a rich-family background, sometimes they do it because they are office workers, sometimes they do it because it's normal.
Most of the time, however, anime characters won't use masu, because it's an anime, and nobody's holding anime accountable for not using polite speech all the time. Because it's for kids. And saying these long words is a pain in the ass. I mean look at this:
The dialogue would never fit in 20 minutes episode if every verb doubled in length. Not to mention it wouldn't fit in speech bubbles in manga either. Only light novels use this stuff. And, also, in real life. This is one of the main reasons anime speech is different from real Japan speech: anime characters don't give a shit about polite speech.
In fact, the most extreme case would be the copula desu です which has all these forms:
- da だ
- desu です
- de aru である
- de arimasu であります
- de gozaimasu でございます
- janai じゃない
- dewanai ではない
- de arimasen でありません
- de gozaimsen でございません
Yes. They all mean the same thing (janai etc. are negative, though). Just different nuances.
So, in conclusion, masu forms mean almost absolutely nothing. If you want to sound like an anime character, you don't want to use masu forms. If you want to sound like you think you know Japanese, you should use masu forms. And if you want to actually know when to use them properly, your best bet is to watch closely and study how and when natives use the masu form and when they don't.