1
Last weekend I went to the mountain to see the snow.
Last weekend I went to the mountains to see the snow.
Use the mountains in the plural when talking about a mountain area: ‘We spent the weekend in the mountains.’
Use the mountain only when referring to one particular mountain: ‘We went to the mountain behind our village.’
In the red sentence, no particular mountain has been identified. The intended meaning is the mountain area in general, so we use the mountains.
2
I like to run on the mountains.
I like to run in the mountains.
I spent three days at the mountains.
I spent three days in the mountains.
Use in the mountains when someone or something is within a mountain area: ‘We went walking in the mountains.’
Use on a mountain when talking about the surface of one mountain: ‘There was snow on the mountain.’
Use at with a particular point: ‘We met at the foot of the mountain.’ ‘They stopped at the top of the mountain.’
Check our grammar lesson on prepositions of place.