Diana Szewczyk

Why do we say Americans, but we say Portuguese, not Portugueses?

It is mostly phonetic and it depends on the final sound of the word. 

Words ending in: -an, -ian, -er, -i require "s" in the plural: 

The Russians, The Koreans, The Americans, The Italians, The New Zealanders.

Words ending in: -ese, -ish, -iss, -ch do not change in the plural:

The Japanese, The Chinese, The British, The Swiss, The French, The Portuguese.

If you are talking about a specific group, the article the must be used.

Most of the Russians in this class are female.

Most of the Japanese in the restaurant are from Kyoto.

If you are talking generally, no article or preposition is needed.

Most Americans speak English. 

Koreans are friendly.

IMPORTANT - Languages do not have articles:

Japanese is difficult / French is easy.