What's the nearest train station to Mont Saint-Michel? Do I need to take a bus to visit it?
How much time does it take? Can I make it in half a day?
Stealing shamelessly from mouviciel's comment (and memory), the correct answer is Pontorson, which is about 9 km from the Mount -- a bit of a hike on foot, but quite doable if you rent a bike.
Also, the article quoted in Nikita's answer is shameless rip off from Wikivoyage, which is particularly annoying since I wrote the original of that text. Time for a nastygram! (Edit: to the person who copied it, not you Nikita!)
Looks like it's Rennes.
There is a TGV that runs from Paris’ Gare Montparnasse to Rennes. It costs about €55.80 and takes a hair over two hours. From Rennes, Keolis buses run to Mont-St-Michel between two and five times per day depending on the season, and most departures are timed to match to the arrival of the TGV in Rennes. The ride takes about an hour and 15 minutes. They also stop at hotels near the site, so keep that in mind if you’re doing an overnight trip. The bus station is immediately outside the north exit of the Rennes train station at the bus terminal building on your right. The bus costs about €11, €8.70 for people under 25 or older than 60, €5.40 for children under 12, and it’s free for children under 4. Bus tickets are sold by the driver when boarding the bus. Another option is to take a TGV train to the Pontorson-Mont St-Michel train station with a stopover in Rennes. During high tourist season there are about four of these trains that leave from Paris each day. The Pontorson train station is no more than 15 minutes from Mont St. Michel. Buses are available from there several times a day; you can get a schedule from the Pontorson train station. There are also two buses daily from Saint Malo to Pontorson (line 17, 1 hour, €2.50), which are timed to connect to buses to Mont St-Michel.
If you book your train in advance, the ticket might be a bit cheaper.