Can we use though and although interchangeably? Somebody told me that the difference is that though cannot be used at the beginning of a sentence. Is that the rule?
Though can definitely be used at the beginning of a sentence, and has a long history of doing so. Consider Isaiah 1:18 from the King James Bible:
Though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
TheFreeDictionary.com cites this usage rule:
Although and though are interchangeable as conjunctions, but to start a clause, use although.
However, this seems overly fussy and prescriptive. Consider the following:
I went ahead and chopped all the wood, though no one told me to.
You left without your luggage, though I can't imagine why.
Those sound to me like perfectly fine English. They would be perfectly fine using although as well. That, to me, feels like interchangeability, pure and simple. In fact, I can think of no examples in which though cannot be used interchangeably with although. Though your mileage may vary.
As conjunctions, although and though are interchangeable. Although is generally considered more formal than though, though both forms appear regularly in both formal and informal writing.
Though is also an adverb, meaning however or nevertheless. In this sense, though is not interchangeable with although, which is only a conjunction.
And in these examples, though is an adverb and hence not interchangeable with although:
This weekend, though, theaters were packed. [Los Angeles Times]
There was another twist to come, though, as Pavlyuchenkova defied her flagging fortunes to win three games in succession. [Independent]
In fact, “though” came before “although.” In the 1300s, before “although” became one word, it was two words —“all” and “though”— with the “all” there to add emphasis to “though (5, 6).”
H.W. Fowler, Modern English Usage, revised edition (1965) offers a concise and generally valid summary of the differences between though and although:
Though, although. The definite differences between the two hardly need stating. They are: first that though can and although cannot be used an adverb, placed last (He said he would come; he didn't, though); and secondly that though is alone possible in the as though idiom. In the use common to both forms, i.e. as a complete conjunction, no definite line can be drawn between them, and either is always admissible. But it is safe to say, in the first place, that though is much commoner, and, secondly, that the conditions in which although is likely to occur are (a) in the more formal style of writing, (b) in a clause that does not follow but precedes the main sentence, and perhaps (c) in stating an established fact rather than a mere hypothesis [examples omitted].
I'm not at all sure about item (c) above, but the rest seems reasonably accurate in the present day. (Fowler made these points originally in 1926, in the first edition of his book.) Especially noteworthy is how Fowler turns on its head the supposed rule about never starting a sentence with though: according to Fowler, the point isn't that you can't start a sentence with though, but that the start of a sentence is a situation where although is relatively likely to occur. Fowler's overriding contention is that though is much more commonly used than although—a state of affairs that was truer in Fowler's day than it is now, to judge from this Ngram chart of though (blue line) versus although (red line) for the years 1700–2000:
The discussion of though and although in Bryan Garner, Garner's Modern American Usage, second edition (2003) indicates how little had changed between 1965 (or indeed 1926) and 2003:
although; though. As conjunctions the words are virtually interchangeable. The only distinction is that although is more formal and dignified, though more usual in speech and familiar writing. In certain formal contexts, however, though reads better. Though serves also as an adverb [example omitted].
Garner doesn't include an example of a formal context in which though reads better than although as a conjunction, though I don't doubt that he could have done so. I am quite familiar with the shift in tone between habitual use of though and habitual use of although. For more than a decade I copyedited textbooks and university press books whose publishers strongly favored although; when I switched to magazine work, I was disconcerted at first by the many instances of though that appeared in manuscripts.
Ultimately, what sold me on though was how much easier it was to work with during copyfitting, the process of cutting lines and altering words to get a story to fit into its available layout space. You wouldn't think that an eight-letter word would be much more difficult to accommodate than a six-letter word—especially when one of the letters is an l—but you'd be wrong. Often I have run into situations where although wouldn't fit even on a heavily kerned line and would leave the line with an unsightly gappiness between the remaining words when it rolled over to the next line, but where though worked fine. (Our house style forbade hyphenation after two letters, so it was all or nothing with although.) Anyway, that's how I came to appreciate the special charms of though, and it's why I've long appreciated the interchangeability of though and although when used as conjunctions.
In terms of SAT, that though cannot be used when starting a sentence is definite not the case.