When should we use desu and imasu/masu? There are few lessons that i get confused as to use which for which. 1)Kyo wa nani o shitai desu ka? (can i use masu here instead of desu?) 2)Kono kurabu ni hairi tai desu ka? (can i use masu here instead of desu?) 3)Ni sen ni nen no Warudo Kappu o oboete imasu ka? (can i use desu instead of imasu?) There's lots more and i'm not sure which one to use and end up with the wrong one. Can someone give me an explaination.

The difference is a matter of deference to the interlocutor. If you speak to someone you do not know, or you need to give some respect (like an elderly, your boss...) it is usual to use the forms in -masu. When you speak in more casual circumstances (friends, colleagues), you can go with desu. In these situations in fact the -masu forms will be a bit too "pedantic". Good day!