Isthmi is plural. There are actually two plural forms. I will list them from singular to plural.
Isthmus - Singular
Isthmi - Plural
Isthmuses - Plural
The plural of isthmus is accepted as either isthmuses or isthmi.
The plural of 'this' is 'these' and the plural of 'that' is 'those'.
An isthmus is a narrow region of land. When an isthmus connects two larger landmasses, it can be viewed as a natural path for land transportation or an obstruction for maritime transportation. Many isthmuses (or, if you prefer, isthmi.) are now cut by canals (Suez in the Sinai, Panama, Corinth) or have been considered for cutting (Kra, Tehuantepec...). Some of the USA's largest cities sit upon isthmi: Seattle and Madison, WI. So, Isthmuses Good: connect places, regions of commerce. Bad: Separate oceans/seas, transportation bottlenecks, Hipsters.
The plural of rose is roses. The plural possessive is roses'.
The plural of 'bunch' is bunches.The plural of 'ant' is ants.The plural of 'batch' is batches.The plural of 'day' is days.The plural of 'chimney' is chimneys.The plural of 'tomato' is tomatoes.The plural of 'umbrella' is umbrellas.The plural of 'donkey' is donkeys.The plural of 'sky' is skies.The plural of 'foot' is feet.The plural of 'show' is shows.
The plural form of can is cans.
applied is does not have a plural but is apply it does have a plural.
The plural is a regular plural, attics.
The plural for the noun loss is losses; the plural possessive is losses'.
pluralsThe word plurals is the plural of plural. As in: I answered the question about plurals to the person who didn't know that the plural of plural is plurals.
pluralsThe word plurals is the plural of plural. As in: I answered the question about plurals to the person who didn't know that the plural of plural is plurals.
Singular ' He/She is Plural 'They are NB Notice they change of verb. NNB 'They is' is NOT correct English.