- where er becomes ie as in preferir -- how do you know which verbs this applies to?

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So one has to know the verb conjugation before knowing that it's not a regular verb --  that would require memorising a lot of verbs just in case the one about to be used might be such ??

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Sorry to tell you this James but you will have to learn which are irregular and it's not just  e>ie changes there are lot of other irregular verb changes.

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James - All '-erir' verbs [apart from the barely used 'enjerir'] change the e to ie:, e.g. sugerir, referir, preferir, diferir, transferir, requerir and a host of others. A similar rule applies to [all !] '-vertir' verbs: convertir, divertir(se), etc. etc.... On the other hand, all '-etir', '-edir' and '-egir' verbs adopt the e>i change.... With help from a CD which enabled words to be listed in reverse-alphabetical order, I examined the contents of my CLAVE dictionary, and posted the results in http://dlmcn.com/ir_verbs.html (the accents are properly represented in http://DLMcN.com/irverbs.doc )

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Hola James,
As Marsha said, there is a bit of memorising to do with irregular verbs in Spanish I am afraid. You can always follow some guide lines as David was suggesting.
In Kwiziq we have all lessons/verbs related to stem changes in the present organised so you can see them all in one go. You can also see lists with the most common verbs undergoing these changes. Have a look (at the bottom of the explanation you can see all related lessons):
Cambio vocálico e> ie in the present
Cambio vocálico e > i in the present
I hope this helps.
Inma