There are certain topics that crop up in nearly all my lessons. I almost always teach pronunciation, especially word stress. Another topic that regularly features is word class, being able to recognise the difference between a noun and an adjective, knowing which type of word to use and being able to transform words.
What do we mean by 'word transformations'
This means we can take one word, for example a verb, and transform it into a different word, perhaps a noun. Adding prefixes such as in- or dis- gives even more words.
Present (v) can be transformed to a presentation (n) given by a presenter (n) who looks very presentable (adj).
Decide (v) transforms into decision (n), decisive (adj) or indecisive (adj).
Why is it useful to transform words?
If we transform words we don't need to learn a completely new set of words, rather we can add to what we already know and expand our vocabulary quickly and easily. Being able to transform words is also a really useful skill in writing, allowing the author to re-write sentences in order to make the text more interesting and more cohesive.
How can I practice this?
The Cambridge exams have a whole question which tests word transformations and you'll also be able to find many word transformations exercises in English course books. You can also test yourself using some of the lists on Quizlet by Adrienne_teacher such as this one here.