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Why is it “geometric” but “theoretical”?

They're all adjectives, so how do you know when to use one over the other? Does it matter? I think it does...

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Then perhaps you'd like to tell us why it matters. –  Barrie England Jun 28 '12 at 17:52
    
Okay... so I think there's importance to it because on the radio, like NPR, they're always saying "the historical record tell us..." and so I just think, "Why aren't they saying historic? How do they know that they should be saying historical instead? They must be right... they're ON NPR..." Then, frustrated and down trodden, I turn off the radio, non the wiser to grammatical usage or the important message of the"historical" record. Sighhh. So that's why I'm asking you guys. So I can finally hear the story with a clear, distraction-free mind! –  OnWords Jun 28 '12 at 22:33
    
Look them up in a dictionary. In some cases, the meaning is nearly the same, but in others, for example "literate" and "literary", there can be huge differences of meaning. –  Peter Shor Jun 29 '12 at 3:48
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marked as duplicate by RegDwigнt Jul 1 '12 at 13:59

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I don't know that there's any general rule about how you would interpret adjectives that sound similar or have common roots.

In the examples you give, "historic" means that an event is important; "historical" means something related to history. So an "historical novel" would be a story about history, that is, a story set in the past. An "historic novel" would be a novel that is great and important. One might very well write a boring historical novel, i.e. a historical novel that is not historic; or a historic novel set in the present, i.e. a historic novel that is not historical.

"Instructive" refers to something that imparts insight or knowledge. "Instructional" describes something intended to be used for teaching. If you say, "Sally's over-reaction to the news was very instructive", you mean that you gained an insight into Sally's psychology from her behavior. If you say, "This book is instructional", you mean that it is intended to teach something, as opposed to being for entertainment or some other purpose.

In general, if you see such similar words, the best thing to do is look them up in a dictionary or at least study the context in which you found them to understand the difference in their meanings.

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Part 1: http://grammar.quickanddirtytips.com/historic-versus-historical.aspx

To summarize, historic has a connotation that carries importance; historical does not have that connotation, and more to do with age.

Part 2: http://ask.metafilter.com/112240/When-to-use-ctive-and-ctional

Sometimes it is easier to comprehend the difference between similar adjectives when they have nouns with them.

Instructional video would be a video that was made so it could possibly teach you something.

Instructive would be more of an after thought of one who watched it; ie. "That video was not very instructive."

Further, according to the above website, OED:

Instructional: Of or pertaining to instruction or teaching; educational.
Instructive: Having the character or quality of instructing; conveying instruction or knowledge.

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