By snap I mean the cursor and input point are drawn to significant points on an object, like the endpoint or midpoint, you see a colored point light up. Actually the cursor snap and indicator happens when you touch an edge anywhere, so you know you are drawing from that edge when you click down. When you draw a line out and get the colored indication of direction... I'd call that a constraint because it is showing how you can constrain the operation to a direction or another edge or face (but I think the tutorials also consider this an inference). Inference is an alignment indicated with other objects' points or direction. Are you speaking about drawing an edge along an edge which is in a group? Otherwise, you don't have to draw another line there, that edge will participate in your new face when you add the remaining edges. It is difficult to see SketchUp edges on a Retina screen. I think this has been improved in 2017 version. Try other screen resolutions. I use a MBP but I don't often use the laptop screen itself for SketchUp. More difficult to see snap indicators too. Zooming may not help. It shouldn't be hard to work off an existing edge. Are you working with a lot of edges off-axis? Sometimes SU forces the axis alignment if it is close to what you are drawing. Are you drawing from or using imported edges, as from AutoCAD? This sometimes introduces edges that are not quite right though they appear to be aligned.