What this suffix does
-grounded describes how solid or realistic a person, idea, or theory is: well-grounded, poorly-grounded.

Suffix · forms adjectives
Describes how sensible or well-supported a person, idea, or theory is: well-grounded (sensible, level-headed) versus poorly-grounded (weakly supported).
Written by Bryan López, English teacher · Updated
-grounded describes how solid or realistic a person, idea, or theory is: well-grounded, poorly-grounded.
Adverb + grounded.
This is not a productive rule you can apply freely: it is a closed set of fixed expressions. Learn them one by one as set phrases.
-grounded
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"Well-grounded" and "well-founded" overlap significantly but have distinct emphases. "Well-grounded" tends to emphasise the connection to something real — the grounding in evidence, experience, or practice. "Well-founded" tends to emphasise the quality and strength of the foundation — how well-built the basis is. A claim can be well-founded on a single compelling piece of evidence; a position is well-grounded when it has been developed through sustained contact with the relevant reality.
In modern usage "well-grounded" is used in both intellectual contexts (a well-grounded argument, a well-grounded theory) and psychological or spiritual contexts (a well-grounded person, a well-grounded response to difficulty). The two uses share the same core metaphor of secure contact with what is real and solid.
A trick to remember it
well-grounded = "firmly grounded" (ground = solid earth, base). Picture a person or idea literally standing on solid ground, not floating in the air.
Exercise 1 · Form the word
Exercise 2 · Pick the right one
"poorly-grounded" describes a theory that is...
The suffix -grounded describes how sensible or well-supported a person, idea, or theory is: well-grounded (sensible, level-headed) versus poorly-grounded (weakly supported). In Spanish it usually maps to fundamentado / arraigado / con los pies en la tierra.
"well" becomes "well-grounded". It is a typical example of the -grounded suffix.
Judges how solid the basis of a claim or fear is: well-founded (justified, based on good evidence) versus ill-founded (baseless).
Judges how solid an argument's logic is: well-reasoned (logically sound) versus poorly-reasoned (weak logic).
Describes how deeply embedded something is: deep-rooted (firmly established) versus shallow-rooted (not well established).
-tion, -ness, -ful, -ly, -able... every ending you need to understand thousands of words at once.