Learn English – Word between rare and common

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.

Non-essential means not absolutely necessary, whereas inessential is more disapproving and can connote trivial.

According to Oxford Dictionaries Online, the prefix non- expresses a neutrally negative sense when added to adjectives, especially when the prefix in- has a special (I would say strongly negative) connotation. Another example is non-human (= not human) and inhuman (= cruel).

http://oxforddictionaries.com/definition/non-?q=non-