What is the rule of thumb for using 'however' in the middle of the sentence? For example:
Some people disagree with this theory, however, as it's never been proven right.
Nothing wrong with a however in the middle of a sentence. You often see a semi-colon in front of the comma, however, instead of a comma, particularly if the second half of the sentence could stand on its own as a complete sentence:
That said, the sentence above sounds awkward. The however should introduce some idea that's contrary to the preceding thought:
This makes more sense. It's another way of saying, "Even though some people disagree with this theory, it's never been proven wrong." (You wouldn't disagree with a theory, unless you thought it was wrong - right?)
I hope this answers your question; however, if it doesn't, you may want to elaborate more on what you're asking.
I don't know about a "rule of thumb", but you must be careful about ambiguous meaning.
Are pretty much equivalent. And that means that:
Could be read as:
... or as ...
The reader has to think pretty hard about the semantic meaning of the sentence, to decide which one you intended (I'd guess the first one, since "Some people disagree with this theory" is consistent with "It's never been proven right"). You might see this as a good thing -- forcing the reader to read closely. But most people aim for easy readability.
You can fix the ambiguity with punctuation:
But, short sentences rock. The clearest thing to do is to break it into short, unambiguous sentences.
However can't be used to hold together two stand-alone statements in a compound sentence, like but can. So, the sentence "Some people disagree with this theory however it's never been proven right" is wrong; it requires but instead.
However is used to "point up" a single stand-alone statement, highlighting an up-coming contradiction or limitation, as Fumblefingers says. But you may artfully place it in the middle (or even at the end!) of that statement, like this: "Some people disagree with this theory, however, as it's never been proven right." See, that's all a single statement, not two bodged together, because the second phrase begins with "as".
And, of course, you may make a compound sentence (properly, using a colon or semicolon) in which the second statement uses however as described above.
Examples above were not very good, because they dealt with sentences not very conducive to "however" in the first place. They also tried to use however to connect two clauses that should be separate sentences. That's not a good way to use "however". Here is a better example:
The Berfschnickel family has been blessed with many excellent athletes. Fred, however, was not one of them.
That flows much better than it does with "however" starting the second sentence:
The Berfschnickel family has been blessed with many excellent athletes. However, Fred was not one of them.
Even worse, joining them into one sentence:
The Berfschnickel family has been blessed with many excellent athletes, however Fred was not one of them.
That just doesn't work. If you want to make just one sentence, use "but", not "however".
However can be used at the beginning and middle of a sentence. Here are some examples below. However, some sentences will need commas.
Beginning with however:
With however in the middle:
Be sure to remember past, present, and future tenses when using words like however.
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