Difference in usage of "regular", "usual", "ordinary", "normal", "common"

Solution 1:

To be perfectly honest, each of these are interchangeable for the most part (given the details explained), and I would guess that certain phrases only sound 'off' because of a lack of familiarity, or, over-familiarity with other constructions: they all indicate a sense of typical circumstance, commonplace.

If we look at the origins as far back as required then we can see quite clearly how we've carried each through:

Regular:

• from Late Latin regularis, according to rule,
• from Latin regula, rod, rule;

Usual:

• from Latin usus, use, from past participle of uti, to use.

Ordinary:

• from Latin ordinarius,
• from ordo, ordin-, order;

Normal:

• from Late Latin normalis,
• from Latin, made according to the square, from norma, carpenter's square;

Common:

• from Latin communitatem "community, fellowship,"
• from communis, "common, public, general, shared by all or many,"

'according to rule', 'order', 'carpenter's square' and 'fellowship / shared by all or many' are all synonymous in trying to say that things are as they should be - any other state is different and, back then, undesirable, and potentially unusable (see: 'to use' above.)

So, each of these refer to a state of normality that today all are equally acceptable, the main difference being selfishness: they now refer mostly to one's self rather than being used in an altruistic fashion (i.e. not necessarily 'for the common good' of society.)

That's my two-penneth, anyway.

Solution 2:

The just to add to the confusion many of these have very specific meanings in technical usage

eg in maths:
regular expression
usual topology
ordinary differential equation
normal form
common factor

Solution 3:

I just saw this answer at Programmers SE which is an answer to a question about a working situation, where he asks if this is normal.

The answer I point to say that it is not normal, but it is common. This is an interesting differentiation of the two terms.

The situation is common in the meaning that similar cases occur many places, and many are in a similar situation.

But it is not normal, as this is not how it should be. It is more like a "freaky" situation (and I think we all agree that freaky is not normal ).