The grading criteria for the Speaking Exam states:
Band 6: is willing to speak at length
Band 7: speaks at length
Band 8: develops topics coherently and appropriately
Band 9: develops topics fully and appropriately
Giving full and clear answers is part of the Fluency and Coherence criteria that the examiner is looking for in your speaking (see the Grading Criteria).
So, you need to give a reason for your belief or argument, particularly in Part 3, which is expected to be an extended discussion of sometimes controversial topics. The question, then, is what language do you need in order to explain and extend your answers? Here are some options:
Because
The easiest and most common conjunction is 'because'. It can be used to link two simple sentences. Here are some examples of 'because' in use:
It is important to protect the environment because our health may depend on it.
For example
To build further, the next simplest way to extend is to use 'for example':
It is important to protect the environment because our health may depend on it. For example, if there is pollution in the air we might face problems with breathing, such as asthma.
Also
To add a further comment, the most common adding conjunction in spoken English would be 'and' or 'also':
It is important to protect the environment because our health may depend on it. For example, if there is pollution in the air we might face problems with breathing, such as asthma. Also, deforestation means there are less trees on the planet, which we need for producing oxygen.
In our writing, we may use alternatives for these conjunctions, such as:
Because = as, for, since
For example = for instance
Also = in addition, additionally, moreover, furthermore
The examples given above are really more formal/ written ways of expressing because, for example and also and might sound a little too formal in the speaking exam. My advice is, keep to the simple examples given for extending your answers but just make sure that you do use them. If you only state an opinion without an explanation, particularly in Part 3, the examiner will probably hesitate and look at you waiting for a further comment. The useful thing about 'because' is that you can pause after saying it. So, make your statement, say 'because' and then pause for a second whilst you think of your further explanation.
If you do not give an explanation or an example, the same as in Writing Part 2, you will lose marks for the Fluency and Coherence section of the marking criteria.
So, remember, 'because, because, because'...
Vocabulary
coherently (adv) = clearly
appropriately (adv) = in a suitable way
controversial (adj) = something not everybody agrees on