'Reichen' and 'genügen'.
Yet another 'are these two synonymous' thread from me.
Are 'das reicht' and 'das genügt' interchangeable, or are there nuances I need to know?
Also, can I use them in response to questions such as 'Noch ein Bier?' when I feel like I've had enough?
Interchangeable in virtually all cases. When you get fed up with a situation, you can say "Mir reicht es!" (I've got enough of this!) just like in english to express your discomfort. It's possible to use "genügt" here also, but very uncommon. When you don't want another beer it's best just to say "Nein, danke." You could also say "Nein, das genügt." but only "Das genügt." would not suffice.
Thanks very much!
One more question: can I tell a person that I'm fed up with them by saying 'du reichst mir'?
Reichen has multiple meanings though, it can be to suffice or to offer/give or to extend to a certain point (which is the original meaning).
Die Zweige des Obstbaums reichen bis in den Garten der Nachbarin.
Gib mir bitte das Salz/Reich mir bitte das Salz.
Es reicht mir!