to show that their is more than one.
"He is good" is correct. 'He' is singular, 'are' is for plurals.
When using the words they and them you have to do so correctly. Both words are used a plurals and this can be difficult. Them is more likely to be the right word to use in a sentence.
you should use you all
It is incorrect I think. Another possible option would me "I have had many chances to meet his students."
When there is more than one of something.
How can you make any sense without plurals and possessives? You have to use the words you need to make yourself understood.
When there is one or more type of cheese included.
The plural of cactus is cacti. Sentence example: Arizona is known for being a state abundant with cacti.
so you can make plurals
3 (oxen, horses and geese)
An apostrophe is used to indicate a possessive noun, either with apostrophe S for most words or an apostrophe alone for plurals ending in S. It is also used in contractions to indicate letters that have been removed, e.g. he is = he's or do not = don't. It is very rarely used to form plurals that cannot be clearly indicated in the normal fashion (How many this's are in this sentence?)
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.