What are auxiliary verbs in Japanese language? Which verbs are auxiliary and how to tell which are auxiliaries, and which are normal?
It looks like the word です is an auxiliary verb. But why? What tells us that it has this function in sentence?
Is the word ます an auxiliary?
I apologize for silly questions.
There are two different definitions of auxiliary verb.
Words like ます, (ら)れる are generally taught as "part of conjugations/forms" in most Japanese-as-a-second-language textbooks, but they are categorized as 助動詞 in monolingual Japanese dictionaries and grammar books. This word class is usually translated as auxiliary verb (or just auxiliary). The list of 助動詞 is relatively small (see below), and you may already be familiar with most of them (as part of "forms"). Generally, 助動詞 is a mishmash of functional words that work by modifying the meaning of the preceding word. Most auxiliary verbs can conjugate, and multiple auxiliary verbs can be "stacked" to say complicated things like "negative-causative-passive". For example, 飲ませられない ("is not forced to drink") is a verb 飲む followed by three auxiliary verbs (causative させる, passive られる and negative ない). Despite its name, a 助動詞 does not necessarily conjugate like a verb; for example, たい conjugates like an i-adjective.
List of Japanese 助動詞
Words like (～て)いる, (～て)みる, (～て)おく are usually called subsidiary verbs (補助動詞), but some people like to call them auxiliary verbs. For details, please see: What is a subsidiary verb?
Regarding です/ます, yes, they are categorized as 助動詞 in Japanese monolingual dictionaries. But I think most beginner Japanese-as-a-second-language textbooks usually regard です as a (polite) copula and ます as a politeness marker.
If you are a beginner, you may be overwhelmed by the number of "forms" in Japanese verbs, but once you reach an intermediate level and start using learning materials written in Japanese, you may want to familiarize yourself with the concept of 助動詞. See also: Why Japanese verb has so many forms?
Without knowing how far you are in your Japanese studies or linguistic studies, it's difficult to know how much detail to provide. But here are two key points to get you started - I suggest you search the site as there are many detailed posts relating to your questions.
です is a copula (in simple terms a copula is a verb which links the subject to an element in the predicate).
Auxiliary verbs have different forms in English and Japanese. In English, they are separate words which modify other words to produce specific functions. In Japanese, auxiliary verbs (助動詞) take the form of declinable words which attach to specific forms of other words. ます is an auxiliary which attaches to the continuative form (連用形) of verbs. For example, the continuative form of 書く is 書き. Now you can add the auxiliary ます to make 書きます.
I have refrained from going into detail here as I believe there are many explanations of these concepts contained in other posts.
