invalid / frail
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invalid
No longer valid, effective, or relevant. Often used in connection with plans, agreements, or rules.
Da der Zeuge nicht erschienen ist, ist die Anklage hinfällig.
Since the witness did not appear, the charges are invalid.
The meaning of 'hinfällig' depends on the context. If it refers to abstract things like plans, rules, or appointments, it means 'ungültig' (invalid). If it refers to a person, it means 'körperlich schwach' (physically weak) or 'gebrechlich' (frail).
Unser alter Plan ist jetzt hinfällig.
Our old plan is now invalid.
In the meaning 'ungültig' (invalid), 'hinfällig' is often combined with the verb 'werden' (to become). The phrase 'hinfällig werden' describes the process of something losing its validity.
Durch die neuen Gesetze werden einige alte Regelungen hinfällig.
Because of the new laws, some old regulations will become invalid.
In the meaning 'gebrechlich' (frail), the word describes a state of severe physical weakness, usually due to age or illness. It is a rather formal and strong word.
Der hinfällige Patient braucht ständige Pflege.
The frail patient needs constant care.
weak
'schwach' and 'hinfällig' can both describe a person as having little physical strength. 'schwach' is the normal, broad word: you can be weak after illness, exercise, or stress. 'hinfällig' sounds stronger and often describes older or ill people who are frail and need help. For brief tiredness, you do not say 'hinfällig', but rather 'schwach' or 'müde' (tired). 'hinfällig' fits when the condition seems longer-lasting and more serious.
Der alte Mann ist sehr hinfällig und braucht Hilfe beim Gehen.
The old man is very frail and needs help walking.
Nach der Grippe fühlt sich Lisa noch schwach.
After the flu, Lisa still feels weak.