I am brand new to this, I was a Cored Draw X3 user.
If I want to duplicate, for instance, parallel horizontal lines at predetermined increments, I only seem to be able to do so by first going to the tool bar selecting OBJECT, then from the drop down selecting TRANSFORM and from the window put in my vertical distance (in this case 10mm).
Then select my line and from the drop down click on APPLY. It does not actually duplicate but it moves my existing line up 10mm.
How can I get a line to replicate itself all the way down the page at 10mm apart and horizontal?
My thanks for any help.

Hi.

Select the object and press Ctrl+D for duplicating.

In the document properties (Shift+Ctrl+D) preferably set mm the default unit (if it's not already) and in the preferences/behavior/steps (Shift+Ctrl+P) you can set a step value of 10 mm.

After those it'd simply be duplicating (Ctrl+D) and pressing an arrow key.

There are various different approaches, depending on what you actually want to achieve. If you want to create a grid to use as a reference when drawing, then Inkscape has grids available via File > Document Properties > Grids. If you want a grid, but made up of real lines, the Extensions > Render > Grids > Grid... tool would be a better option.

Tiled clones could also work, but would need a bit of fiddling as they work with percentages of the bounding box size, not with absolute values.

If, however, you want a general approach to duplicating *any* object at a specific vertical or horizontal distance, Lazur's approach works, as does the following:

1) Select the object and press Ctrl-D to duplicate.
2) In the toolbar, find the spinbox for the Y coordinate. Check that the units are "mm" (drop down further along the toolbar), then just put the cursor in the Y field, at the end of the existing value, and type "+10". Hit enter.