(Written Oct. 19 2015)
NOUN-ADJECTIVE PAIRS
Some learners of English make the mistake of confusing the noun and the adjective forms of some words. For example, "efficient" is an adjective and the noun is "efficiency".
I have written a list of more than 300 such word pairs, which you can download HERE. If you print out this list, you will be able to cover the noun column with a piece of paper and test yourself to see if you know the related adjective, or vice versa. That list just contains the word pairs that I could think of but, of course, there are more word pairs that are not on that list.
Since you are preparing yourself for the Speaking test, not just the Writing test, you also need to make sure that you can pronounce these words correctly. If you want to hear the pronunciation of a word, just put it into the box at http://www.howjsay.com. Let's look at one example, "nation" (noun) and "national" (adjective). In that particular example, the "a" in "nation" is pronounced like the "a" in "cake" but the "a" in "national" is pronounced like the "a" in "cat". So you see that you really do need to check the correct pronunciation of many of these word pairs. "Nature" (noun) and "natural" (adjective) has that same pronunciation pattern.
If you don't have much time, perhaps just learning some of the word pairs in my downloadable list would be good enough for your IELTS preparation. But if you would like to understand this topic a little more, there are places on the internet that explain it quite well. The main thing that they explain is that there are certain "rules" or patterns that are usually (but not always) followed. One good web page on this topic is http://www.gingersoftware.com/content/grammar-rules/adjectives/ and the related pages, http://www.gingersoftware.com/content/grammar-rules/adjectives/forming-adjectives/ and http://www.gingersoftware.com/content/grammar-rules/adjectives/lists-of-adjectives/ .
The following web page also has a short list of common noun-adjective pairs - http://www.englishw.com/AdjectivesMain.php.
You also should understand that not every noun has a related adjective, and sometimes an adjective exists without a related noun.
Adjectives Derived from Verbs
Another thing that many people don't know is that there are many adjectives that are derived from a verb and these adjectives usually follow a certain pattern. There is usually, but not always, a noun that is also derived from these verbs. For example, there is a verb, "to tire". The two main adjectives from that verb are "tiring" and "tired". These adjectives are originally the "participles" of the verb, which are used in some of the verb tenses such as, "I need to sit down for a while because this work is tiring me", and "My work had tired me before I arrived home." "Tiring" is called the "present participle" and "tired" is called the "past participle" when these words are used as parts of a verb tense.
When used as adjectives, these participles are used this way – "tiring" is used as an adjective to describe what creates tiredness but the statement does not include information about who is made tired by that something. For example, "This is very tiring work."
As an adjective, "tired" is used when it refers to who is made tired but information about what created the tiredness is not included. For example, "He is tired", which is grammatically just like, "He is happy" and "He is hungry".
There is also another adjective, "tiresome", which is similar (but not exactly the same) in meaning to "boring". The noun that goes with the verb "to tire" is "tiredness".
Speaking of boring, the verb "to bore" acts in the same way as the verb, "to tire". When used as a verb we can say, "He bores me". When the participles are used as adjectives, we can say - "This is a boring topic" (= "This topic is boring") and, "I'm bored" (which does not say what caused me to be bored). The noun that goes with this verb is "boredom".
If this grammar is too complex for you to understand, don't worry. The main thing is to teach yourself when the "... ing" form of the word should be used, and when the "... ed" form should be used. Many Chinese people make mistakes with this usage because the Chinese language uses just one word for both forms. For example, if you say, "I'm boring" then you are saying, "I make people feel bored"! A similar statement is, "I feel boring", which means, "I feel that I am boring to other people". It is grammatically possible to make those statements, but people rarely admit to such a personal trait – they usually mean to say, "I'm bored" = "I feel bored".
In the IELTS test, this topic could be included under "Grammar", or it could be included under "Vocabulary". I would say it is slightly more a vocabulary topic than a grammar topic for the purpose of grading an IELTS test, especially for those noun-adjective pairs that are not derived from a verb, such as "efficiency" and "efficient".
----------------------------------------------------