This exercise has students examine the various forms of the prefix in- to discover its meaning and to discover the reasons it has different forms.
What does the morpheme in- mean?
What does the morpheme ir- mean?
What does the morpheme im- mean?
impossible improbable imbalance
What does the morpheme il- mean?
Do all of these - in-, ir-, im-, il- - mean the same thing?
Why are there all of these different forms of the same morpheme? (Think about the sounds that follow the prefix.)
Some interesting notes about IN: There are actually THREE prefixes in-. We'll combine the first two in- prefixes, since they have roughly the same meaning. One comes from Old English, the other from Latin, but they are both equivalent to the English word in. This first in- can have the broad meaning in (inland, entrust), and can also function as a general intensive (incantation). Depending on the phonetic context, it can take several different forms, including im-, em-, and en-.
The other in- is a Latin negative prefix, equivalent to English un-. Some of its other forms are im-, il-, and ir-. This prefix is used to indicate a negative or the absence of something.
The word infamous does not mean 'not famous'; it means 'having an extremely bad reputation' and hence 'wicked'. This is because it's not an English formation of in- and famous, but a borrowing of Latin infamis 'without a reputation', formed from the sense of famis in Latin meaning 'a reputation'.
The word invaluable also uses the negative prefix, but the explanation for the unusual sense of the word is that valuable is not the current meaning 'having considerable monetary or moral worth', but an obsolete meaning 'capable of being valued'. So etymologically, invaluable means 'not capable of being valued'; compare the semantics of priceless. It's the shift of the sense of valuable that makes it confusing.
Finally, inflammable, which means 'capable of being set on fire; combustible', is one that often confuses people, because the in- here is not the negative prefix. It is our first prefix, having the meaning in, so inflammable has an etymological meaning like 'into flames' (it's cognate with the English word enflame, which doesn't give anyone any trouble). But because of the fear that some people would mistake the prefix in inflammable for the negative one, the word flammable was coined. And that's the reason why flammable and inflammable mean the same thing.