when is てます used over ています if you are supposed to be talking "polite"


I understand that てる and てます are basically shortened versions of ている and ています,
but if you are supposed talking "formally"/"polite" why would you use a
shortened version?




てる and てます are very common and safe in ordinary workplaces when you are
_speaking_ Japanese. You can even combine it with honorific verbs and say
something like 見てらっしゃいます. But they are generally avoided when you are
_writing_ something formally. Also, when you are giving a formal speech with a
microphone in front of dozens of people, it's safer to avoid this contraction
even if it's spoken.

In general, you cannot explain an expression's register using a single axis.
We need to use different adjectives that are related but different, such as
"honorific", "polite", "written", "formal", "stilted" and "pompous". If I have
to choose a single adjective to describe てる/てます, I would choose "colloquial",
which is related to but not exactly the same as "informal".

