I was thinking about the words "today" and "tonight" and trying to think of what "to" means in these words. A cursory Google search turns up no results for "to-" in any prefix lists.
Learn English – Is “to” a prefix in English
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"Inflammable" is derived from the verb inflame, which comes from in- and flame. The OED identifies the prefix as in-2, indicating the second definition of the prefix, rather than the third, which is the negation which is what you believed it to be from. I quote the right definition below:
used in combination with verbs or their derivatives, [...] with the senses ‘into, in, within; on, upon; towards, against’, sometimes expressing onward motion or continuance, [...] . (emphasis mine)
To inflame something is to set it on fire–i.e. to use motion to cause something to be in flames.
"Invaluable" does come from in- expressing negation, and thus it means not able to be valued. However, this can be interpreted two different ways—one, it is so worthless that it has no value, or two, it is so valuable that we can not put a value on it—like the concept of there being no finite number that is larger than the rest—you can always add one. The common meaning is #2, but the OED recognizes both definitions.
Neither of these examples are exceptions—the first is misleading because the two prefixes look identical, the second can be understood in two separate ways. The best way to figure these out—have a good dictionary at hand.
The formation of verbs in many Indo-European languages follows the following rule
prefix + root verb
Examples
- English incoming, outgoing
- German einkommen (income), ausgehend (outgoing)
- Latin inīre (to come in), exīre (to go out)
German for instance still follows this system very closely and has a small number of ubiquitous prefixes which fall into two categories:
- The so called separable1 prefixes (ab- "off", an- "on", auf- "up", aus- "out", ein- "in" etc.), which indicate an ongoing action, a movement or a direction.
- And the so called inseparable1 prefixes: (be- (be), ent-, er-, ge-, mis-, ver- and zer-) which indicate a completed action (differentiated according to the type of outcome: neutral, successful, failed...).
The English be- prefix is clearly the same as the German be- and is therefore a remnant of its Germanic ascendancy.
It is actually of the same origin as the verb to be which linguists have traced back, through Proto Germanic to the reconstituted Proto Indo European root *bheu-, *bhu- meaning "to grow", "to turn into" or " to become"2.
Ultimately most of the English words starting with a "be-" can be traced back to this notion of "to turn into"3.
The general form is:
be + [quality]
and the corresponding meaning is:
to turn into + [quality].
Let's illustrate all this theory with a few simple examples:
- to befriend somebody => to turn somebody into a friend.
- to beget something => to make something supplied, produced.
- to besot somebody (besotted) => to turn somebody into a sot (a dummy).
- to bewitch somebody => to make somebody possessed by a spell.
- to bedazzle somebody => to make him confused (see also bewilder)
These ones are slightly more difficult:
- to behold: the original meaning of to hold (OE healdan, German halten) is to keep. But to keep by actually keeping an eye on, to watch over. => So to behold is to make something watched.
- to bedevil someone => to make someone feel like in Hell.
- to believe something => to make something dear (loved). See also German glauben (ge + lieben) as well as Dutch geloven.
- to belong to someone => to make something go along with somebody.
As usual there are a few exceptions or look-alikes that don't fit into the template
- to behead => sometimes the "be" is categorised as privative but you can also interpret it as "turn into a head (and not much else)". The question being what do you sever: the head or the body ?
- between. It is not a verb. The "be" is akin to "by" and the "tween" part is akin to "two".
But their existence is not sufficient to belie the general theory outlined above.
Note 1:
German verbs starting with a separable prefix form their past participle by inserting a "ge-" between the prefix and the root. For instance:
Infinitve aus-gehen "to go out" => past participle aus-ge-gangen "gone out".
The meaning of "ge-" in this role is actually the same as the meaning of "ge-" taken as an inseparable prefix: it indicates that an action is complete.
Inseparable prefixes already have the meaning of a completed action. Therefore the past participle form of the verbs starting with this kind of prefixes does not need an additional "ge-".
Note 2:
The Proto Indo European root *bheu-, *bhu- also enters into the composition of the German equivalent of to be:
- ich bin "I am" and
- du bist "you are".
Note 3:
In German the past participle of the verb kennen (to know) is gekannt (known) but it has a close relative in bekannt, which also means "known".
However, bekannt is the past participle of "bekennen" which means "to confess, to acknowledge" - that is to say "to turn into a known thing" whereas the simple un-prefixed kennen verb means "to known".
In English, there used to be a similar word beknown and we still use unbeknownst (see German unbekannt - credits @OregonGhost's comment).
Best Answer
to (a prep.) : Old English to "in the direction of, for the purpose of.....