It's fine to use fineliners here, but of course we should always think about why you might be struggling with a certain aspect of a tool. Remember that when we add line weight, we do so by going back over an existing mark, not by purposely varying the weight of the original stroke. This is usually done at the end of the drawing, to clarify specific overlaps. Also keep in mind that this use of line weight is always meant to be quite subtle - going back over an existing mark with a ballpoint pen isn't going to add a lot of obvious weight, but it will add enough for the viewer's subconscious to pick up on. By seeing the slight change in relative thickness, the viewer's subconscious is able to make better sense of what they're looking at.

Looking at your form intersections, you're largely doing a great job of drawing these forms such that they feel cohesive and consistent in the same space, and you're also demonstrating a very well developing grasp of how they all relate to one another with the contour lines you've used to define their intersections. One point worth mentioning, is actually about your use of line weight. You've focused on those localized areas nicely, but one issue is that there is a visible jump from thick to thin where that line weight stroke ends. Remember that line weight should be applied without pressing down too hard, and with a confident stroke, to ensure that the marks taper naturally as shown here to help the stroke blend back into the original mark more seamlessly. A stroke will taper when the pen is still in motion as it lifts off the page.

Moving onto your object constructions, I'm honestly really pleased with your work across the board here. You're showing a lot of patience and care as you pin down all the necessary subdivisions to place the various forms in a specific location relative to their neighbours. Nothing here appears to be guesswork or estimation - if there's something you need to pin down, you're clearly going to all the lengths to do it.

In addition to this, you're also constructing your forms such that they feel solid and believable, although I am noticing some issues in some of your box constructions. For example, if we look at this soap dispenser and try and look at all the lines that ought to be parallel to one another in 3D space (like the vertical edges), they appear to be somewhat more inconsistent, especially towards the top. This suggests to me that you may need to focus more on some freely rotated boxes (with line extensions included, like the 250 box challenge) as a greater part of your warmups to help improve the consistency of your convergences. Always remember to think about how each line relates to all those it's meant to run parallel with (in 3D space), so you can keep their relationships consistent. Those enclosing boxes are at the end of the day the bedrock of our constructions.

That said, I am pleased with the fact that even where the initial box enclosure comes out a little skewed or warped, you still stick to it instead of trying to correct that mistake after the fact. You show a good deal of respect for the forms you've committed to, and that is a good habit to have.

So! All in all, I am very pleased with your work here. Just be sure to mind those box constructions and the convergence of your 3d-parallel-lines. I'll happily go ahead and mark this lesson as complete. Keep up the great work.