1. Five nouns of foreign origins that retain their original plural endings:
- analysis (singular) : analyses (plural)
- phenomenon (sg.) : phenomena (pl.)
- criterion (sg.) : criteria (pl.)
- datum (sg.) : data (pl.)
- oasis (sg.) : oases (pl.)
All of these nouns aren't originally from English, but rather from Latin or Greek, and they kept their foreign plural forms.
2. Three words that are always in the plural form:
- fireworks: you cannot buy one firework - it's always plural
- goods: meaning merchandise, it's always plural
- outskirts: the environment outside the town, it's always plural