I drew a long curved line with the spline command, and when I revisit the line later and select it, the square handles have turned to circles, and there are LOADS of the, making them near impossible to edit as there are so many handles to move, and then the line ends up all wobbly as I've not managed to line them all up right. 

Also, AutoCAD doesn't seem to be recognising them with other commands. If I try and trim one such sline against another, it won't work half the time. Also sometimes they won't offset - or even weirer, only a part of them will offset, or they might offset in part, but only on one side.

No idea how this is happening - I'm a relative newbie to AutoCAD as I'm a student so hopefully it's nothing too complicated! 

Thank you in advance!

:)

Trimming or extending polylines and splines is not allways good.

Other ways : there are ending grips on your spline, select your spline and move slowly your mouse on one ending point,

see what message appears with 4 editing options for you to "work" your spline.

Tell us what happens.

With the old releases of AutoCAD I use to definte the ending tangencies on the command line but with my 2013 I have to come back on the spline ending grips to set it OK.