der Grund
reason
The cause, occasion, or justification for an action, event, or state.
Der wahre Grund für seine Kündigung ist immer noch unklar.
The real reason for his resignation is still unclear.

reason / ground
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reason
The cause, occasion, or justification for an action, event, or state.
Der wahre Grund für seine Kündigung ist immer noch unklar.
The real reason for his resignation is still unclear.

'Grund' (reason/ground) is the basis for words like 'gründen' (to establish something on a foundation), 'begründen' (to give a reason), and 'gründlich' (thorough, going down to the bottom). The idea of a base or foundation connects all the meanings.
Bitte begründen Sie Ihre Entscheidung.
Please justify your decision.
You can easily recognize the different meanings of 'Grund' from context: 'Grund für' is the cause, 'Grund und Boden' is land ownership, and 'Grund des Meeres' is the lowest part of a body of water.
Was ist der Grund für deine schlechte Laune?
What is the reason for your bad mood?
When 'Grund' (reason) describes a cause, it is almost always used with the preposition 'für' followed by the accusative. The pattern is: der Grund + für + accusative object.
Es gibt keinen Grund für Panik.
There is no reason to panic.
In the meaning of 'Ursache' (cause), 'Grund' appears in many fixed expressions, for example 'aus diesem Grund' (for this reason), 'aus gutem Grund' (for good reason), or 'Grund zur Freude haben' (to have reason to be happy).
Er hat seine Arbeit gekündigt, und das aus gutem Grund.
He quit his job, and for good reason.
The expression 'im Grunde' (basically) is a modal adverb and means 'im Wesentlichen' (essentially) or 'eigentlich' (actually). It is used to reduce a statement to its core.
Im Grunde genommen ist die Idee gut, aber die Umsetzung ist schwierig.
Basically, the idea is good, but the implementation is difficult.
To describe the lowest part of a body of water, the preposition 'auf' is always used with the dative: 'auf dem Grund' (at/on the bottom).
Am Grund des Sees fand man einen alten Schatz.
An old treasure was found at the bottom of the lake.
The expression 'von Grund auf' (from the ground up) means 'from the beginning', 'completely', or 'fundamentally'. It is often used in connection with learning, building, or renovating.
Er hat das Handwerk von Grund auf gelernt.
He learned the craft from the ground up.
actually / really
'eigentlich' and 'im Grunde' can both express what something really is or what it is at its core. 'eigentlich' is a single adverb and very often fits in everyday sentences. 'im Grunde' is a fixed expression with 'Grund' and sounds a little more explanatory: you use it to summarize the main point. They can be confused because both are often meant in the sense of 'when viewed realistically'. But not every meaning of 'der Grund' fits here; as a normal noun, 'der Grund' means a reason or cause.

Im Grunde war die Reise gut geplant, nur das Wetter hat Probleme gemacht.
Basically, the trip was well planned; only the weather caused problems.
Die Fahrerin sagt, dass die Strecke eigentlich nicht lange dauert.
The driver says that the route actually does not take long.