Sam C wrote:

sorry if my question wasn't clear.  the advice was When used as the subject, use "I,"  whether you have another person as part of the subject or not. i wondered why? sam

In most cases, it's impossible to say why a particular word (i.e. a particular sound) has a particular function in a language. For instance, why do the different sounds "ego", "ich", "je", "io", "yo", and "I" signify "the person speaking, as the subject of the verb" in different languages? We might postulate a common root, thirty thousand years ago – a "person speaking" noise – but that doesn't tell us why the noise isn't "suitcase" or "harlequin" instead.

All we can say is that "this particular noise does have this particular function, in this particular language". We can then add that another noise, which usually has a different function – "me" – is sometimes substituted for the first noise, by some speakers, in some situations (e.g. your "me and Bill are going to the shops").

If enough speakers insisted on using the second noise instead of the first noise, the first noise would become obsolete (cf. "thou"). However, there is usually some resistance to such change, as it's more trouble for most speakers to replace word A, which usually has function X, with word B, which usually has function Y, than simply to use word A for X and word B for Y.

That's not to say that the substitution in this instance ("me" for "I") might not have its own function, which some speakers find useful. For instance, in BrE at least, "I" has acquired a slightly "overbearing" air, in some contexts (cf. the Queen's "My husband and I"). By fronting "me" in place of "I", in "me and Bill...", for instance, perhaps the speaker unconsciously puts himself in the background: perhaps "Bill and I" would seem too Queenly, to some speakers.

(The problem for non-native speakers, though, is that native speakers are very tolerant of each other's grammatical slips – in fact, most people don't notice them – but immediately notice if those grammatical slips are combined with a foreign accent.)

<End of pontification.>

MrP

Grammar Geek wrote:

Yikes! I'm sorry, but "Me and mom" as the subject is NOT acceptable. You will sound like an overly-enthusiastic 8-year-old boy if you say "Me and mom" while speaking...

MrPedantic wrote:

The problem for non-native speakers, though, is that native speakers are very tolerant of each other's grammatical slips – in fact, most people don't notice them – but immediately notice if those grammatical slips are combined with a foreign accent.