A good way to check this is with an etymology dictionary.  For example:

> **[rudiment][1]** (n): *1540s, from Middle French rudiment (16c.) or directly from Latin rudimentum "early training, first experience, beginning, first principle," from rudis "unlearned, untrained" (see rude).*

So *rudimentary* knowledge is something taught to someone who is completely *unskilled*.  For example, a *rudimentary* carpentry skill might be how to hammer a nail in straight (without hitting your thumb), or how to glue together two pieces of wood.

Both "fundamental" and "foundation" share the same Latin root *fundare* ("to lay a bottom or foundation") so both words mean much the same thing.  In contrast with *rudiment*, *foundational/fundamental* knowledge forms the basis for everything else in that field.  Again, using carpentry as an example, this might be how to select good wood for a particular project, how to create proper joins, how to properly build load-bearing structures, how to use and maintain your tools, and so on.

Obviously there is some overlap between *rudimentary* and *fundamental*, since you need to teach the basics to someone untrained.  But the intent is different -- it sounds much better to say you have a strong *foundation* in a discipline, rather than saying you have picked up the *rudiments* of that discipline.


  [1]: https://www.etymonline.com/word/rudiment