I can draw lines in Inkscape (with Bezier tool or freehand line tool), but they appear as path objects and when I save it, in the svg file they are also appear as path. My question is: How can I draw and save lines as line and not as path with Inkscape?
TLDR: You can't really, but you can kind of*
In Inkscape lines, or shapes (except for rectangles**), are generally constructed using the concept of vector paths, to which fills and strokes are applied. That's how the software works. There's no Line Segment Tool in Inkscape, like the one in Illustrator.
*It is possible to link or embed raster images in Inkscape and other vector image editors, but these will remain as raster images made of pixels. It's also possible to rasterize objects made in Inkscape using Edit > Make Bitmap Copy, but again these will then be made of pixels, and not vector.
*Also, you could use underscore characters to make a line, which would of course be a text object, rather than a path. But then again, strictly speaking, fonts are still made of vector paths.
**Also possible is to create a long rectangle, and fill it. Technically not a path as such, but an SVG rect object, as you will see if you examine the XML code.
Edit: further to the comments with user287001, it would seem there are other possibilities. Illustrator has a Line Segment Tool, and a line made with it, and exported as SVG, will create an SVG line object. Another possibility is to hand code the line segment. Shouldn't be too hard to hack it. And strangely enough Inkscape can see these SVG elements if you open such an SVG in Inkscape.
Here's an example SVG line segment that seems to work in Inkscape, you just can't create one with a tool, nor edit the end points with a tool using the GUI. You can rotate it, and stretch it though.
<line x1="37.770248" y1="32.135063" x2="80.329521" y2="18.314875" id="line1" style="fill:none;stroke:#231f20;stroke-width:0.48102528;stroke-miterlimit:10" />
Inkscape seems to create XML code for a path even if you draw a straight line segment with the pen tool in straight line mode. I guess some programming is needed either for converting path commands to SVG line commands or for creating a drawing tool which creates internally SVG line expressions.
Unfortunately I cannot show such Inkscape extension program and even less I can make one. But check this discussion in the sister site: https://stackoverflow.com/questions/9102563/turn-svg-path-into-line-segments
A quick search popped out also this code. https://jsfiddle.net/fq9n7f76/18/
I must admit I cannot see is it valid, but it seems to ouput polylines. Hopefully you can read and speak this language better.
Some CAD programs can export the wanted SVG lines. I have checked that at least LibreCAD does it because some CAM-tools want it. LibreCAD is freeware. Check, if you can do your drawing work there.