You only separate "ins" into "in das" when you want to put some emphasis on "das". In that case, you would translate something like "in das Restaurant" as "to THAT restaurant".

It depends on the context ;) For one sentence alone is 'Ich gehe in das Restaurant.' OK. but not OK is: Ich habe Hunger. Ich gehe jetzt in das Restaurant. here you have to write 'ins Restaurant'. but OK would be: Ich habe Hunger. Ich gehe jetzt in das Restaurant am Rathaus. ;) some hints, how and when to use contractions you can find here: http://www.canoo.net/services/OnlineGrammar/Wort/Artikel/Prep-Art/Gebrauch.html?lang=en