Hey MathsNerd, I will be doing the MTeach so I can help you out.
A sub-major is an area of study in which you have to complete at least four units of study (with no more than two at first year level)
So for example, say you wanted to teach Mathematics. At the bare minimum, you need to do four units at university level. Only two maximum will be taken into account at first year level, and the other two can be from second or third year.
So if you do three first year maths units, and then only one second year unit.. Even though it's four units, it's technically only three when considering your eligibility as per that condition.
Technically they classify it as 'study taken over two years with no more than two at first year'.. but the latter condition only exists (no more than two at first year level).
For further reference, when you apply via VTAC for the MTeach, VTAC checks against these guidelines in regards to your eligibility:
http://www.vit.vic.edu.au/SiteCollectionDocuments/PDF/1672_Specialist-Area-Guidelines-Nov08.pdfSub-major: A total of half a year of successful full-time higher education study, usually comprising sequential discipline studies taken over two years, e.g. a part in each of the first and second years of study, or equivalent study. In most programs this equates to four units, with no more than two at first year level.
So overall whilst you are doing your degree you do need to look to the future and consider what you may want to teach because you need to complete the requirement to even be considered to teach that area.