Think of the morpheme as the logical unit.

@linguoboy:

So its a word, but it can only appear as a part of another word, right...?

@Dan_ad_nauseam:
What do you mean by that?

Attached to but not necessarily part of. "They've" is still really two words (which is why we write it the way we do), it's just that one of them is in an abbreviated form that can't appear on it's own. You can't say *"'Ve they gone?" even though there's no real difference in meaning between have and 've.

This is a very different case from, for instance, the -hood in brotherhood or the -ian in Canadian.

One way to identify a morpheme is to ask if it adds logical meaning.

I'm afraid I don't understand what definition of "logical" you're using here. Is it possible to add "illogical meaning" to something?

Dan_ad_naseum, that's a pretty good rule, but one has to be careful.

Exception:
Flammable indicates something is easily set fire. "in"+"flammable"="inflammable" Inflammable means something easily set fire to. So, in=?

If parsed in the order:

In- + flame -> inflame
Inflame + able -> inflammable

In-'s status as a morpheme can be identified.

An example would be the "il" in "illogical."

In- + logical -> illogical (/n/ changing to /l/ for phonetic reasons)

"In-" adds the meaning "not."

But take an example like Spanish lógico "logical" > lógica "logical". What "logical meaning" has been "added" here? And if none has, then what status does -o/-a have in these words?