A stem may also be defined as the part of the word that remains unchanged throughout its paradigm. The stem of the paradigm  h e a r t y - h e a r t i e r - (the) h e a r t i e s t) is  h e a r t y-. It is a free stem, but as  it consists of a root morpheme and an affix, it is not simple but derived. Thus, a stem, containing one or more affixes is a d e r i v e d stem.

Roots are main morphemic vehicles of a given idea in a given language at a given stage of its development. A root may also be regarded as the ultimate constituent element which remains after the removal of all functional and derivational affixes. It is a common element of words within a word-family.

Thus, - h e a r t - is the common root of the following series of words: h e a r t,

h e a r t e n, d i s h e a r t e n, h e a r t i l y, h e a r t l e s s, h e a r t i n e s s, s w e e t h e a r t, k i n d - h e a r t ed, etc.

The root word h e a r t is unsegmentable. The morphemic structure of all other words in this word - family is obvious - they are segmentable as consisting of at least two distinct morphemes. They may be further subdivided into: 

1) those formed by affixation or affixational derivatives consisting of a root morpheme and one or more affixes: h e a r t e n, h e a r t i l y, h e a r t l e s s, h e a r t i n e s s.

2) compounds, in which two or more stems simple or derived are combined into a lexical unit: s w e e t h e a r t, h e a r t - s h a p e d, or 

3) derivational compounds where words of a phrase are joined together by composition and affixation: k i n d - h e a r t e d. This last process is also called phrasal derivation ( kind heart) + - ed)

To be continued.