Difference between 者{もの} and 人{ひと}

Question

I never heard 者{もの} alone, just for work profession.

1. Both means person right?
2. Is 者{もの} old-fashioned, when used alone?

Example:

[...] 居る者

Answer 1

Not necessarily old-fashioned, but standalone 者 does sound stiff.

It's commonly found in military settings or in legal documents, where everything is written in an objective manner:

• 違反した者は、6か月以下の懲役もしくは100万円以下の罰金に処す。

It's also used as a humble expression of 人 in formal business settings. Using 人 is clearly inappropriate in the following sentences:

• はじめまして、私は田中という者です。
• 申しわけありません、会社の者は、ただいま全員外出しております。
• 担当の者を呼んで参ります。

Although it may sound quite insolent, a person with a higher status can directly addresses their people like this:

• 反論がある者はいるか？

Moreover, standalone 者 is frequently found in serious written articles in general, as an synonym of 人, as long as honorific expressions are not necessary. I did Wikipedia search and found that "する者は" is used more frequently than "する人は" there.

• ～だと考える者もいれば、そう考えない者もいる。
• 生きている者にはすべて、等しく1日に24時間が与えられている。
• ナレーションをする者のことを、ナレーターと呼ぶ。
• [*]今日は、大阪市に住んでいる者にインタビューをします。: This is rude. Use 大阪市に住んでいる人に (neutral) or 大阪市に住んでいる方に (politer)

Answer 2

者(もの) and 人(ひと) means "person" and as you said 者 isn't uesd alone, it is used as a　part of a word like ばか者. And a part of a word "者" is often read as しゃ like 消費者、被害者、旅行者、出席者　etc.

Answer 3

Yes, both mean "person", and there is no difference in meaning; only in nuance.

者 {もの} is used in formal contexts, whereas 人 {ひと} is much more casual. For example, you would most likely use "あの人" in conversation, yet on certificates (including mine for the JLPT), "人" is replaced by "者" in all contexts (except kanji compounds). 〜もの referring to people is sometimes used as a suffix for a handful of words in everyday language, but when used on its own, it's reserved exclusively for formal context, where its meaning maps 1:1 with "人".

It's not really that 者 is old-fashioned -- it's still used today in formal situations -- but it is uncommon (though it's always been that 人 has been the more common and casual in comparison).