Pluralising ‘axe’ (or ‘ax’ in American English) is simple. You just add -s to it to become ‘axes’, referring to the heavily bladed tool used for cutting and splitting wood. But what about ‘axis’? Pluralising it is dramatic — to say the least.
The reason is that ‘axis’ is one of the English words with Greek or Latin origin. Pluralising them follows patterns that are so irregular that one can easily get confused using them. For instance, while we simply add ‘s’ to bus, minus, tax, etc., to realise their plurals (buses, minuses, and taxes), we do not just dump the ‘s’ at the end of ‘axis’. Rather, it is the ‘i’ that changes to ‘e’. In other words, the plural of ‘axis’ is not ‘axises’ but ‘axes’. This means it shares a plural with ‘axe’.
The guys operate only in this axis. (Correct)
They guys operate in two or three axises. (Wrong)
The guys operate in two or three axes. (Correct)
Note that ‘crisis’ follows a similar path. For one, its plural is not ‘crisises’!
The company is facing a major crisis. (Correct)
The company is facing some major crisises. (Wrong)
The company is facing some major crises. (Correct)
Criterion, alumnus
These are two other words with a Greek/Latin background. They also turn plural in an irregular way. ‘Criterion’ is the singular form of ‘criteria’, while ‘alumnus’ is that of ‘alumni’:
What criteria was used to share the materials? (Wrong)
What criterion was used to share the materials? (Correct)
What criteria were used to share the materials? (Correct)
Mr Gabriel is an alumni of the University of Lagos. (Wrong)
Mr Gabriel is an alumnus of the University of Lagos. (Correct)
Mr Gabriel and his wife are alumni of the University of Lagos. (Correct)
Hope you remember that the plural of ‘phenomenon’ is not phenomenas but phenomena (sometimes ‘phenomenons’).
Index, cactus: Double-plural words
Still in the family of nouns with Greek/Latin backgrounds, ‘index’ and ‘cactus’ stand out for having two plurals. This is how they manifest:
The index is not correct. (Correct)
The indices are not correct. (Correct)
The indexes are not correct. (Correct)
The cactus in the compound always attracts visitors’ attention. (Correct)
The cacti in the compound always attract visitors. (Correct)
The cactuses in the compound always attract visitors. (Correct)