A. Some words, however, remain the same for plural.

• deer – deer
• sheep – sheep
• salmon – salmon
• mackerel – mackerel

B. An “a” ending

• phenomenon – phenomena
• criterion – criteria

C. A change from um to a

• curriculum – curricula
• medium – media
• memorandum – memoranda

D. A change of end letters to es or i for foreign words that have become part of English.

• Virus – viruses
• octopus – octopuses
• circus – circuses
• stimulus – stimuli
• syllabus – syllabuses or syllabi
• terminus – terminuses or termini