Only is no ordinary word. Not only an adverb, it is also an adjective. Only too happy to be useful, it also serves as a conjunction, only you shouldn’t really use it this way unless you’re being slightly informal.
(With examples from the Chrestomanci novels, Buffy and Discworld)
As an adverb, “only” refers to a limitation or restriction.
only (adverb) 1. without others or anything further; alone; solely; exclusively. 2. no more than; merely; just.
Cat learnt that Chrestomanci’s power was limited only by his vulnerability to silver.
“Hey!” Xander objected. “Are the freaky nut cookies for slayers only or something?”
In these examples, “only” is highlighting elements of limitation and restriction: Chrestomanci’s power is limited solely by contact with silver; the eating of cookies was (apparently) being restricted to Buffy (and possibly Faith and Kendra...). If someone or something is being excluded, or if a limitation of some sort exists, then “only” will be needed in order to establish these facts.
But wait, there’s more. A great deal more. As an adverb, “only” can also be used to show the consequences of an action.
only (adverb) inevitably, although unfortunate or undesirable.
“Ah yes,” Giles said, voice laced with sarcasm. “However appealing whole scale demon massacre may be, it will only hasten the approach of the apocalypse.”
only (adverb) with the negative or unfortunate result that.
Angua took a long and twisting path through the city back streets, only to find Nobby right behind her when she turned around.
In these examples, “only” is implying a sense of inevitability - that there are unavoidable consequences of the characters’ actions. Going around madly killing demons will make an apocalypse more likely; no matter how hard Angua tries, she won’t be able to shake Nobby off. If “only” is taken out of these examples, they lose somewhat of that inevitability.
In adjective form, “only” once more refers to uniqueness or exclusivity.
only (adjective) 1. being the single one or the relatively few of the kind. 2. having no sibling or no sibling of the same sex. 3. single in superiority or distinction; unique; the best.
Buffy stared into the darkness, betraying no fear of the horror that awaited her. “I’m the only one who can stop it.”
“I wish I were an only child,” Cat whispered.
This was a woman who could stop an avalanche with a stern look. This was the one and only Granny Weatherwax.
If something is unique in some way, then “only” helps to emphasise that fact.
When used as a conjunction, “only” serves as a replacement for words such as “but” or “except”.
only (conjunction) but (introducing a single restriction, restraining circumstance, or the like).
“I was going to turn his nose into a potato,” Nanny Ogg confessed, “only he flinched and I got ’is cat instead.”
This is generally considered to be an informal use of the word, and is best limited to dialogue or first person narrative.
“Only” also finds its way into the idiom “only too”, meaning “incredibly” or “extremely”.
Cat was only too eager to escape Chrestomanci Castle for an afternoon.
It can also emphasise the inevitability of an event, usually in a negative sense.
However, he knew it was only too likely that something would happen to prevent his escape.
However useful “only” may be, it can be an unfortunate victim of ambiguity. It’s important to look at how you’ve structured your sentence to make sure your meaning hasn’t been subverted or confused by the placement of “only”. Take this example:
Vimes had long accustomed himself to the fact that Sybil only served meat on Fridays.
This could mean Sybil is either limiting Vimes’s meat-eating to Fridays or that Vimes is going to eat nothing but meat on Fridays. You can avoid confusion by placing “only” directly before or after the word that needs to be limited. So, if Fridays are going to be Vimes’s sole meat-eating days, we’d let “only” follow directly after “Fridays”:
Vimes had long accustomed himself to the fact that Sybil served meat on Fridays only.
Or, if Sybil is going to serve up nothing but meat on Fridays, then we’d place the “only” directly before “meat”:
Vimes had long accustomed himself to the fact that Sybil served only meat on Fridays.
This way, there’s no confusion and the meaning is clear.
“Only” is a useful word which will help you in highlighting the importance or uniqueness of any given person, object or action. It can thrive in sarcastic environments and can sneakily change the meaning of sentences depending on its placement. If you use it properly, then it’s only too happy to be helpful.