I'm just adding some extra info, but Moini posted the direct answer to your question.
I'm guessing you must have used Trace Bitmap in the default mode (the first option, Brightness Cutoff). Assuming that's the case, these are the best steps, in my opinion.
- import the image
- run Trace Bitmap
- drag either the trace results or the original image off to the side and out of the way
- select all the trace results (drag the Selection tool around the whole thing)
- Path menu > Break Apart (everything goes black)
- hold the Shift key while you click the black chip on the palette (this adds a stroke) (but doesn't look like anything happened)
- click the tiny X chip on the far left end of the palette (which removes the fill)
Now you can see all the paths, and you can delete the ones that you don't need. I don't think you need to combine them, to be able to cut them. But if you do, Moini told you how to do it, and still see the holes in the middle.
I don't usually suggest for newbies to use Outline mode. I think it's better for newbies to learn how to see the actual paths, as paths rather than outlines. If you never learn how to manipulate the paths, you'll always run into problems which you can't fix. But if you learn the basics, so you can get yourself out of trouble if it happens, then you're better off in the long run.
Another option, which might work pretty well for this particular image, is to use the Centerline Trace extension. It's a special trace that only gives you a single line, and you don't have to go through and delete half the paths. Unfortunately, it doesn't come installed with Inkscape, so you have to install it....which isn't exactly straightforward. If you want to try, it's here:
https://github.com/fablabnbg/inkscape-centerline-trace Also, it tends to create strange effects around the intersection of paths. Since this particular image doesn't really have any intersections, it should work pretty well. But anyway, you can try it and see what you think - assuming you can get it installed.
But personally, I would trace it with the Pen tool. I made a video for someone recently, showing the basics of tracing with the Pen tool. Ok here, read this message. It's someone else who wants to do the same thing:
https://forum.inkscapecommunity.com/index.php?topic=1582.msg10664#msg10664Note that grouping and combining, while they sometimes seem to do the same thing, don't do the same thing. You always need one or the other, and almost never need to do both. Grouping is about objects. Any kind of objects can be grouped. Combining is all about paths. Only paths can be combined.
The place where people get confused is because a path is one kind of object. So paths can be grouped or combined. And you can do both, but it's usually a waste of time to do both.
In this image, you needed to break apart. And if you did need to put them back together, you would use Combine, and not Group. But I think you're better off without combining or grouping in this case. Sorry, I'm in a bit of a hurry at the moment. Feel free to ask questions if I skipped over anything too fast.