"He eats a sandwich."

Translation: Er isst ein Sandwich.

[hodgdon]

Is there a rule when one should use "belegtes Brot" instead "Sandwich" to say sandwich?

[wataya]

"Belegtes Brot" is more general than "Sandwich". If you say "Sandwich" in Germany you specifically mean something like that: http://de.wikipedia.org/wiki/Sandwich "belegtes Brot" can also to refer to something like this: http://www.chefkoch.de/forum/2,59,319081/CdTS-50-am-24-Mai-2007.html (you have to scroll down a bit)

[Alscott]

I wrote 'einen Sandwich' which it said was okay - but it also said 'ein Sandwich' is correct.

Why? That seems to indicate 'Sandwich' can be either masculine or neuter. Is that possible?

[wataya]

You're right. 'Sandwich' can be both masculine and neuter.

[arneheij]

Sigh.

[sabanerox]

Is there a difference when to use it, or I can use it masculine or neuter the way I want it

[wataya]

It's a regional thing. If you perform the grand average, the neuter form is more common.