Learn English – Should I use “even though” or “although” in the following sentence

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.

From Subscrioption-Only LDOCE:

Yet is used to say that something has not happened or a situation has not started to exist, or to ask if something has happened:

It isn't time to go yet.

Have you seen him yet?

**Still* is used to say that an earlier situation has not changed:

This system of naming is still used today (NOT yet used today).

I still don't understand.

So I'd say in that sentence it's better to use still, rather than yet, because you had been thinking, and it hasn't changed so far, and you are still thinking.