[susan53]

Hi Mark,
Because is always used with cause/reason and so with results/consequences. Try it the other way round - it doesn't work.

So basically it's a matter of which way round you want to present the two pieces of information. If you order them cause, connective + result then you'll link with so - as in your example : I needed eggs, so I went to the store.

Change the order and it becomes result, connective + cause : I went to the store because I needed eggs.

There's a lot more that could be said about their grammar (because is a subordinator and therefore the position of the because clause could change, so a co-ordinator or sentence connective which will always come in mid-position between the two pieces of info) and use (in formal language so would tend to be replaced by therefore etc), but I don't think it's relevant to your example. Here, the only thing the student needs to know is the difference in meaning.

Hope that helps.