Compound subjects joined by the word 'and'are plural since there is more than one of them. Double subjects joined by 'or'or 'nor' match to a verb based on the status of the subject closest to the verb.
Compound subject with a plural verb
Rock and grass combine to make Gardens-by-the-Bay amazing.
'Rock and grass' is a plural subject formed by two singular words. Do not get confused and use 'combines' for the verb because the individual subjects are singular.
Non-compound double subject functioning as a singular subject
Depending on where you look, rock or grass dominates your view.
Since the subjects are joined by 'or' they do not automatically become plural because there are two of them.
