The word 'octopi' is the plural form of the noun 'octopus'.
The alternate plural form 'octopuses' is also generally accepted.
The word octopuses is a plural for for the singular octopus. Another plural form is octopi. Both plurals are accepted and in most dictionaries.
The word "octopus's" is a singular possessive noun, a word for one octopus that shows something in the sentence belongs to that octopus.Example: The octopus's color changed to blend into its background.There are two accepted forms for the plural of octopus:octopi (possessive form octopi's)octopuses (possessive form octopuses')Examples: The octopi's tank contained four octopi.The octopuses' tank contained four octopuses.
Octopi and octopuses are both accepted plural forms for the singular octopus.
The word team is singular; the plural form is teams.
These is plural, this is singular
Lance is singular. Lances is plural.
The word 'Phenomenon' is singular. The plural form is 'Phenomena'.
There are two accepted forms for possessive singular nouns ending in s:Add an apostrophe (') after the existing s at the end of the word: octopus'Add an apostrophe s ('s) after the existing s at the end of the word: octopus'sThere are two accepted plural forms for octopus, they are octopuses and ocotopi.The plural possessive forms are octopuses' and octopi's.
Dictionaries now show both octopuses and octopi as the plural for octopus. In fact, when I spell checked this answer, the word octopi was highlighted for misspelling, the word octopuses was not. Either is correct.
The word metastasis is singular, metastases is plural.
The word "ground" can be both singular and plural. Singular: "The ground is wet." Plural: "The grounds of the park are well-maintained."
Louse is singular. The plural form is lice.