で at the beginning of a sentence

Question

I hear native speakers using で or was it でー when speaking a lot at the beginning of a sentence. Judging by context, I think it translates to either "and" or "therefore"　or "so" or "because of that", but I might be wrong. An example usage I have heard (if i recall) would be as follows: 1. で、言いたいのは何？ 2. これは他人のものですよ。で、取っちゃダメ

I've only ever heard it in speech and never seen in in writing. So the question is, in casual written context, would it be written as
で、言いたいのは何　or
でー、言いたいのは何　or
でー言いたいのは何 (notice that there is no comma) or
で言いたいのは何 (notice that there is no comma)

I know it sounds (very) trivial, but I'm just curious which is the correct form

I am tagging it as slang at the moment, but do let me know if there are better tags

Answer 1

Judging by context, I think it translates to either "and" or "therefore"　or "so" or "because of that", but I might be wrong.

You are not wrong. 「で」 is an extremely common conjunction meaning "and", "and then", "for that reason", etc.

It is an informal/colloquial form of 「それで」、「そういうわけで」, etc.

Since 「で」 is informal, it is not used in formal writing or speech, but it is often used in informal writing and speech.

When it is used in writing, it is mostly in the form of 「で、～～～」 with a comma. In very casual texting among young people, however, the comma may be dropped.

「でー」 or 「でぇ」 with elongation is quite common in speech, but not very in writing.

Finally, though it is informal, it is not slang by any stretch.