When you don't finish a subject within the regular school year, you have options! In a public, private, or alternative school, the only option is usually a big "W" on your transcript - for "Withdraw." But independent homeschoolers have many more options available!
You can give 1/2 credit instead of 1 credit
If it's a class that doesn't need a whole credit, then you can award 1/2 credit and just be done with it. For example, if you are working on Logic, and you really wanted to get through a whole credit class but just didn't, this is a great way to just be done! Instead of finishing, just give the class 1/2 credit on your transcript.
You can add the course content to another course
If you didn't finish a class, but the content can be added to another course, then you can roll the content together. For example, if you had a lot of work in health, but you didn't finish the book you were using, you can consider rolling in that content into your PE class. That might change your title to PE with Health, to show that the content was combined.
You can give credit when you finish the course
If you are working all year, but at a slower pace, you can decide to award credit when you are done - even if you don't finish until the following year. For example, if you were working on a fine art credit, and it took you 18 months to complete the 120-180 hours to get a high school credit, you might put it on the transcript like this: Survey of Fine Art, 1.0 credit, completion date 11/2014.
You can change the name of the class
For algebra, for example, if you worked all year, but didn't get the whole math book done, you can indicate by the class title that you got halfway through the book. One year you might give 1 whole credit for Algebra 1 A, and the next year you could give 1 whole credit for Algebra 1 B. That will show that you worked all year, but you allowed a slower pace.
You can drop the class completely
If a class isn't required by anyone, and the child hasn't finished the class, you can always drop the class without recording that on the transcript. In my own homeschool, our plan was to take Latin 4, but we never even got off the ground with our plan. I didn't mention that anywhere on the transcript at all.
You can evaluate what "finish" means
Count textbooks first. You can be done when you have completed 80% of the curriculum. You can give a whole credit when you finish the textbook, no matter how many hours the textbook, required. You can be done when you combine delight-direct learning with textbook or schoolish learning, and you have 120-180 hours of work. Count textbooks first. You can be done when you have achieved the next level, based on some sort of achievement test. You can be done when the class isn't necessary.
I hope that helps!