Fundamental is an adjective alone, but is a noun in plural form. Fundamental means the essence or basics of something that is intangible, such as a skill, language, concept, etc. Foundation, on the other hand, is a noun, and can be used to refer to the basis or support for something else whether it is tangible (the foundation of a building), or intangible (the foundation of democracy). It's only a subtle difference, but a native speaker would find it sounding awkward if you were to say something like the "fundamentals of a house".