then president vs then-president

The 1st.

Impossible to tell without context.

The context is as follows:

Then Prime Minister / Then-Prime Minister Lee Kuan Yew of Singapore was disappointed when Singapore had to separate from Malaysia.

In that context , Then-Prime Minister is correct.

It isn't a formulation that is used much these days. We used to see it back in the 1960's and 70's.

In that sentence, I would use the hyphen so there’s no confusion that it means a previous position rather than “next” in time.

Here's the rule.

Then is a conjunctive adverb. Here's a handy list of them: http://www.yourdictionary.com/index.php/pdf/articles/144.conjunctiveadverblist.pdf

And here's what the Chicago Manual of Style says about them:
http://www.chicagomanualofstyle.org/16/images/ch07_tab01.pdf
(see page 3, under the section entitled: "adverb not ending in ly + participle or adjective")

That table is particularly helpful as it gives you all of the hyphenation rules.

Now, keep in mind that there are other style manuals out there. See: http://calstatela.libguides.com/c.php?g=767431&p=5504995

Probably the best-known in English are Chicago, APA, and maybe Turabian, which I used in high school 40 years ago. I currently own the AP (Associated Press) style manual which is more for how to write for a newspaper. It's fascinating how it's all put together.

Anyway, my point here is that these aren't arbitrary rules - there's actual thought behind them.

Thanks to all of you.