Not all nouns that end in -s are plural.
my favorite subject.
Some nouns ending in -s are never plural. Others are sometimes singular and sometimes plural, depending on the meaning.
Here are some nouns ending in -s that can be singular:
acoustics | gymnastics | physics |
biceps | kudos | politics |
blues (music) | linguistics | robotics |
crossroads | mathematics | series |
economics | news | statistics |
If you’re unsure about the number of a particular word in a particular context, check a dictionary.